diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4f4c1efa05f..c71d7449bd9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 ### Added
 
 - We added a query parser and mapping layer to enable conversion of queries formulated in simplified lucene syntax by the user into api queries. [#6799](https://github.com/JabRef/jabref/pull/6799)
+- We added some basic functionality to customise the look of JabRef by importing a css theme file. [#5790](https://github.com/JabRef/jabref/issues/5790)
 
 ### Changed
 
diff --git a/src/main/java/org/jabref/Globals.java b/src/main/java/org/jabref/Globals.java
index a03fbfc5ac9..8663a282b13 100644
--- a/src/main/java/org/jabref/Globals.java
+++ b/src/main/java/org/jabref/Globals.java
@@ -13,7 +13,6 @@
 import org.jabref.gui.util.DefaultFileUpdateMonitor;
 import org.jabref.gui.util.DefaultTaskExecutor;
 import org.jabref.gui.util.TaskExecutor;
-import org.jabref.gui.util.ThemeLoader;
 import org.jabref.logic.exporter.ExporterFactory;
 import org.jabref.logic.importer.ImportFormatReader;
 import org.jabref.logic.journals.JournalAbbreviationRepository;
@@ -80,7 +79,6 @@ public class Globals {
     private static KeyBindingRepository keyBindingRepository;
 
     private static DefaultFileUpdateMonitor fileUpdateMonitor;
-    private static ThemeLoader themeLoader;
     private static TelemetryClient telemetryClient;
 
     private Globals() {
@@ -95,12 +93,10 @@ public static synchronized KeyBindingRepository getKeyPrefs() {
     }
 
     // Background tasks
-    public static void startBackgroundTasks() throws JabRefException {
+    public static void startBackgroundTasks() {
         Globals.fileUpdateMonitor = new DefaultFileUpdateMonitor();
         JabRefExecutorService.INSTANCE.executeInterruptableTask(Globals.fileUpdateMonitor, "FileUpdateMonitor");
 
-        themeLoader = new ThemeLoader(fileUpdateMonitor, prefs);
-
         if (Globals.prefs.shouldCollectTelemetry() && !GraphicsEnvironment.isHeadless()) {
             startTelemetryClient();
         }
@@ -146,8 +142,4 @@ public static void stopBackgroundTasks() {
     public static Optional<TelemetryClient> getTelemetryClient() {
         return Optional.ofNullable(telemetryClient);
     }
-
-    public static ThemeLoader getThemeLoader() {
-        return themeLoader;
-    }
 }
diff --git a/src/main/java/org/jabref/JabRefGUI.java b/src/main/java/org/jabref/JabRefGUI.java
index 721f6a686fd..fd228609a3d 100644
--- a/src/main/java/org/jabref/JabRefGUI.java
+++ b/src/main/java/org/jabref/JabRefGUI.java
@@ -48,6 +48,7 @@ public JabRefGUI(Stage mainStage, List<ParserResult> databases, boolean isBlank)
         this.bibDatabases = databases;
         this.isBlank = isBlank;
         this.correctedWindowPos = false;
+
         mainFrame = new JabRefFrame(mainStage);
 
         openWindow(mainStage);
@@ -86,7 +87,7 @@ private void openWindow(Stage mainStage) {
         root.getChildren().add(JabRefGUI.mainFrame);
 
         Scene scene = new Scene(root, 800, 800);
-        Globals.getThemeLoader().installCss(scene, Globals.prefs);
+        Globals.prefs.getTheme().installCss(scene);
         mainStage.setTitle(JabRefFrame.FRAME_TITLE);
         mainStage.getIcons().addAll(IconTheme.getLogoSetFX());
         mainStage.setScene(scene);
diff --git a/src/main/java/org/jabref/gui/JabRefDialogService.java b/src/main/java/org/jabref/gui/JabRefDialogService.java
index 0f45d987b09..292486b146a 100644
--- a/src/main/java/org/jabref/gui/JabRefDialogService.java
+++ b/src/main/java/org/jabref/gui/JabRefDialogService.java
@@ -38,7 +38,6 @@
 import org.jabref.gui.util.BackgroundTask;
 import org.jabref.gui.util.DirectoryDialogConfiguration;
 import org.jabref.gui.util.FileDialogConfiguration;
-import org.jabref.gui.util.ThemeLoader;
 import org.jabref.gui.util.ZipFileChooser;
 import org.jabref.logic.l10n.Localization;
 import org.jabref.preferences.JabRefPreferences;
@@ -69,21 +68,19 @@ public class JabRefDialogService implements DialogService {
     private static final Duration TOAST_MESSAGE_DISPLAY_TIME = Duration.millis(3000);
     private static final Logger LOGGER = LoggerFactory.getLogger(JabRefDialogService.class);
     private static JabRefPreferences preferences;
-    private static ThemeLoader themeLoader;
 
     private final Window mainWindow;
     private final JFXSnackbar statusLine;
 
-    public JabRefDialogService(Window mainWindow, Pane mainPane, JabRefPreferences preferences, ThemeLoader themeLoader) {
+    public JabRefDialogService(Window mainWindow, Pane mainPane, JabRefPreferences preferences) {
         this.mainWindow = mainWindow;
         this.statusLine = new JFXSnackbar(mainPane);
         JabRefDialogService.preferences = preferences;
-        JabRefDialogService.themeLoader = themeLoader;
     }
 
     private static FXDialog createDialog(AlertType type, String title, String content) {
         FXDialog alert = new FXDialog(type, title, true);
-        themeLoader.installCss(alert.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(alert.getDialogPane().getScene());
         alert.setHeaderText(null);
         alert.setContentText(content);
         alert.getDialogPane().setMinHeight(Region.USE_PREF_SIZE);
@@ -116,7 +113,7 @@ protected Node createDetailsButton() {
 
         // Reset the dialog graphic using the default style
         alert.getDialogPane().setGraphic(graphic);
-        themeLoader.installCss(alert.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(alert.getDialogPane().getScene());
         alert.setHeaderText(null);
         alert.setContentText(content);
         alert.getDialogPane().setMinHeight(Region.USE_PREF_SIZE);
@@ -139,7 +136,7 @@ public <T> Optional<T> showChoiceDialogAndWait(String title, String content, Str
         choiceDialog.setHeaderText(title);
         choiceDialog.setTitle(title);
         choiceDialog.setContentText(content);
-        themeLoader.installCss(choiceDialog.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(choiceDialog.getDialogPane().getScene());
         return choiceDialog.showAndWait();
     }
 
@@ -148,7 +145,7 @@ public Optional<String> showInputDialogAndWait(String title, String content) {
         TextInputDialog inputDialog = new TextInputDialog();
         inputDialog.setHeaderText(title);
         inputDialog.setContentText(content);
-        themeLoader.installCss(inputDialog.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(inputDialog.getDialogPane().getScene());
         return inputDialog.showAndWait();
     }
 
@@ -157,7 +154,7 @@ public Optional<String> showInputDialogWithDefaultAndWait(String title, String c
         TextInputDialog inputDialog = new TextInputDialog(defaultValue);
         inputDialog.setHeaderText(title);
         inputDialog.setContentText(content);
-        themeLoader.installCss(inputDialog.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(inputDialog.getDialogPane().getScene());
         return inputDialog.showAndWait();
     }
 
@@ -184,7 +181,7 @@ public void showErrorDialogAndWait(String message, Throwable exception) {
         ExceptionDialog exceptionDialog = new ExceptionDialog(exception);
         exceptionDialog.getDialogPane().setMaxWidth(mainWindow.getWidth() / 2);
         exceptionDialog.setHeaderText(message);
-        themeLoader.installCss(exceptionDialog.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(exceptionDialog.getDialogPane().getScene());
         exceptionDialog.showAndWait();
     }
 
@@ -193,7 +190,7 @@ public void showErrorDialogAndWait(String title, String content, Throwable excep
         ExceptionDialog exceptionDialog = new ExceptionDialog(exception);
         exceptionDialog.setHeaderText(title);
         exceptionDialog.setContentText(content);
-        themeLoader.installCss(exceptionDialog.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(exceptionDialog.getDialogPane().getScene());
         exceptionDialog.showAndWait();
     }
 
@@ -262,7 +259,7 @@ public Optional<ButtonType> showCustomDialogAndWait(String title, DialogPane con
         alert.getButtonTypes().setAll(buttonTypes);
         alert.getDialogPane().setMinHeight(Region.USE_PREF_SIZE);
         alert.setResizable(true);
-        themeLoader.installCss(alert.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(alert.getDialogPane().getScene());
         return alert.showAndWait();
     }
 
@@ -287,7 +284,7 @@ public <V> void showProgressDialog(String title, String content, Task<V> task) {
             task.cancel();
             progressDialog.close();
         });
-        themeLoader.installCss(progressDialog.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(progressDialog.getDialogPane().getScene());
         progressDialog.show();
     }
 
@@ -310,7 +307,7 @@ public <V> Optional<ButtonType> showBackgroundProgressDialogAndWait(String title
         alert.getButtonTypes().setAll(ButtonType.YES, ButtonType.CANCEL);
         alert.getDialogPane().setMinHeight(Region.USE_PREF_SIZE);
         alert.setResizable(true);
-        themeLoader.installCss(alert.getDialogPane().getScene(), preferences);
+        preferences.getTheme().installCss(alert.getDialogPane().getScene());
 
         stateManager.getAnyTaskRunning().addListener((observable, oldValue, newValue) -> {
             if (!newValue) {
@@ -319,7 +316,7 @@ public <V> Optional<ButtonType> showBackgroundProgressDialogAndWait(String title
             }
         });
 
-        Dialog<ButtonType> dialog = () -> alert.showAndWait();
+        Dialog<ButtonType> dialog = alert::showAndWait;
 
         return showCustomDialogAndWait(dialog);
     }
diff --git a/src/main/java/org/jabref/gui/JabRefFrame.java b/src/main/java/org/jabref/gui/JabRefFrame.java
index 28204b87a42..a7590558a1c 100644
--- a/src/main/java/org/jabref/gui/JabRefFrame.java
+++ b/src/main/java/org/jabref/gui/JabRefFrame.java
@@ -121,7 +121,6 @@
 import org.jabref.gui.undo.UndoRedoAction;
 import org.jabref.gui.util.BackgroundTask;
 import org.jabref.gui.util.DefaultTaskExecutor;
-import org.jabref.gui.util.ThemeLoader;
 import org.jabref.logic.autosaveandbackup.AutosaveManager;
 import org.jabref.logic.autosaveandbackup.BackupManager;
 import org.jabref.logic.citationstyle.CitationStyleOutputFormat;
@@ -162,7 +161,6 @@ public class JabRefFrame extends BorderPane {
 
     private final SplitPane splitPane = new SplitPane();
     private final JabRefPreferences prefs = Globals.prefs;
-    private final ThemeLoader themeLoader = Globals.getThemeLoader();
     private final GlobalSearchBar globalSearchBar = new GlobalSearchBar(this, Globals.stateManager, prefs);
 
     private final FileHistoryMenu fileHistory;
@@ -180,7 +178,7 @@ public class JabRefFrame extends BorderPane {
 
     public JabRefFrame(Stage mainStage) {
         this.mainStage = mainStage;
-        this.dialogService = new JabRefDialogService(mainStage, this, prefs, themeLoader);
+        this.dialogService = new JabRefDialogService(mainStage, this, prefs);
         this.stateManager = Globals.stateManager;
         this.pushToApplicationsManager = new PushToApplicationsManager(dialogService, stateManager, prefs);
         this.undoManager = Globals.undoManager;
diff --git a/src/main/java/org/jabref/gui/preferences/AppearanceTab.fxml b/src/main/java/org/jabref/gui/preferences/AppearanceTab.fxml
index 2eb988a44e2..e2f57285195 100644
--- a/src/main/java/org/jabref/gui/preferences/AppearanceTab.fxml
+++ b/src/main/java/org/jabref/gui/preferences/AppearanceTab.fxml
@@ -1,13 +1,19 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <?import javafx.geometry.Insets?>
+<?import javafx.scene.control.Button?>
 <?import javafx.scene.control.CheckBox?>
 <?import javafx.scene.control.Label?>
 <?import javafx.scene.control.RadioButton?>
 <?import javafx.scene.control.Spinner?>
+<?import javafx.scene.control.TextField?>
 <?import javafx.scene.control.ToggleGroup?>
+<?import javafx.scene.control.Tooltip?>
 <?import javafx.scene.layout.HBox?>
 <?import javafx.scene.layout.VBox?>
+
+<?import org.jabref.gui.icon.JabRefIconView?>
+
 <fx:root prefWidth="650.0" spacing="10.0" type="VBox" xmlns="http://javafx.com/javafx/11.0.1"
          xmlns:fx="http://javafx.com/fxml/1" fx:controller="org.jabref.gui.preferences.AppearanceTabView">
     <fx:define>
@@ -28,4 +34,18 @@
     <Label styleClass="sectionHeader" text="%Visual theme"/>
     <RadioButton fx:id="themeLight" text="%Light theme" toggleGroup="$theme"/>
     <RadioButton fx:id="themeDark" text="%Dark theme" toggleGroup="$theme"/>
+    <HBox alignment="CENTER_LEFT" spacing="4.0">
+        <RadioButton fx:id="customTheme" text="%Custom theme" toggleGroup="$theme"/>
+        <TextField fx:id="customThemePath" prefWidth="350.0" disable="${!customTheme.selected}"/>
+        <Button onAction="#importTheme" disable="${!customTheme.selected}"
+                styleClass="icon-button,narrow"
+                prefHeight="20.0" prefWidth="20.0">
+            <graphic>
+                <JabRefIconView glyph="OPEN"/>
+            </graphic>
+            <tooltip>
+                <Tooltip text="%Browse"/>
+            </tooltip>
+        </Button>
+    </HBox>
 </fx:root>
diff --git a/src/main/java/org/jabref/gui/preferences/AppearanceTabView.java b/src/main/java/org/jabref/gui/preferences/AppearanceTabView.java
index 1d97c4e0acf..21dd8c94bf1 100644
--- a/src/main/java/org/jabref/gui/preferences/AppearanceTabView.java
+++ b/src/main/java/org/jabref/gui/preferences/AppearanceTabView.java
@@ -6,6 +6,7 @@
 import javafx.scene.control.CheckBox;
 import javafx.scene.control.RadioButton;
 import javafx.scene.control.Spinner;
+import javafx.scene.control.TextField;
 
 import org.jabref.gui.util.IconValidationDecorator;
 import org.jabref.logic.l10n.Localization;
@@ -20,6 +21,8 @@ public class AppearanceTabView extends AbstractPreferenceTabView<AppearanceTabVi
     @FXML public Spinner<Integer> fontSize;
     @FXML public RadioButton themeLight;
     @FXML public RadioButton themeDark;
+    @FXML public RadioButton customTheme;
+    @FXML public TextField customThemePath;
 
     private final ControlsFxVisualizer validationVisualizer = new ControlsFxVisualizer();
 
@@ -49,8 +52,18 @@ public void initialize() {
 
         themeLight.selectedProperty().bindBidirectional(viewModel.themeLightProperty());
         themeDark.selectedProperty().bindBidirectional(viewModel.themeDarkProperty());
+        customTheme.selectedProperty().bindBidirectional(viewModel.customThemeProperty());
+        customThemePath.textProperty().bindBidirectional(viewModel.customPathToThemeProperty());
 
         validationVisualizer.setDecoration(new IconValidationDecorator());
-        Platform.runLater(() -> validationVisualizer.initVisualization(viewModel.fontSizeValidationStatus(), fontSize));
+        Platform.runLater(() -> {
+            validationVisualizer.initVisualization(viewModel.fontSizeValidationStatus(), fontSize);
+            validationVisualizer.initVisualization(viewModel.customPathToThemeValidationStatus(), customThemePath);
+        });
+    }
+
+    @FXML
+    void importTheme() {
+        viewModel.importCSSFile();
     }
 }
diff --git a/src/main/java/org/jabref/gui/preferences/AppearanceTabViewModel.java b/src/main/java/org/jabref/gui/preferences/AppearanceTabViewModel.java
index 6602791e261..b2f3e1638b0 100644
--- a/src/main/java/org/jabref/gui/preferences/AppearanceTabViewModel.java
+++ b/src/main/java/org/jabref/gui/preferences/AppearanceTabViewModel.java
@@ -10,10 +10,15 @@
 import javafx.scene.control.SpinnerValueFactory;
 
 import org.jabref.gui.DialogService;
-import org.jabref.gui.util.ThemeLoader;
+import org.jabref.gui.util.FileDialogConfiguration;
+import org.jabref.gui.util.Theme;
 import org.jabref.logic.l10n.Localization;
-import org.jabref.preferences.JabRefPreferences;
+import org.jabref.logic.util.StandardFileType;
+import org.jabref.model.strings.StringUtil;
+import org.jabref.preferences.AppearancePreferences;
+import org.jabref.preferences.PreferencesService;
 
+import de.saxsys.mvvmfx.utils.validation.CompositeValidator;
 import de.saxsys.mvvmfx.utils.validation.FunctionBasedValidator;
 import de.saxsys.mvvmfx.utils.validation.ValidationMessage;
 import de.saxsys.mvvmfx.utils.validation.ValidationStatus;
@@ -24,21 +29,28 @@ public class AppearanceTabViewModel implements PreferenceTabViewModel {
     public static SpinnerValueFactory<Integer> fontSizeValueFactory =
             new SpinnerValueFactory.IntegerSpinnerValueFactory(9, Integer.MAX_VALUE);
 
+    private static final String EMBEDDED_DARK_THEME_CSS = "Dark.css";
+
     private final BooleanProperty fontOverrideProperty = new SimpleBooleanProperty();
     private final StringProperty fontSizeProperty = new SimpleStringProperty();
     private final BooleanProperty themeLightProperty = new SimpleBooleanProperty();
     private final BooleanProperty themeDarkProperty = new SimpleBooleanProperty();
+    private final BooleanProperty themeCustomProperty = new SimpleBooleanProperty();
+    private final StringProperty customPathToThemeProperty = new SimpleStringProperty();
 
     private final DialogService dialogService;
-    private final JabRefPreferences preferences;
+    private final PreferencesService preferences;
+    private final AppearancePreferences initialAppearancePreferences;
 
-    private Validator fontSizeValidator;
+    private final Validator fontSizeValidator;
+    private final Validator customPathToThemeValidator;
 
-    private List<String> restartWarnings = new ArrayList<>();
+    private final List<String> restartWarnings = new ArrayList<>();
 
-    public AppearanceTabViewModel(DialogService dialogService, JabRefPreferences preferences) {
+    public AppearanceTabViewModel(DialogService dialogService, PreferencesService preferences) {
         this.dialogService = dialogService;
         this.preferences = preferences;
+        this.initialAppearancePreferences = preferences.getAppearancePreferences();
 
         fontSizeValidator = new FunctionBasedValidator<>(
                 fontSizeProperty,
@@ -53,55 +65,96 @@ public AppearanceTabViewModel(DialogService dialogService, JabRefPreferences pre
                         Localization.lang("Appearance"),
                         Localization.lang("Font settings"),
                         Localization.lang("You must enter an integer value higher than 8."))));
+
+        customPathToThemeValidator = new FunctionBasedValidator<>(
+                customPathToThemeProperty,
+                input -> !StringUtil.isNullOrEmpty(input),
+                ValidationMessage.error(String.format("%s > %s %n %n %s",
+                        Localization.lang("Appearance"),
+                        Localization.lang("Visual theme"),
+                        Localization.lang("Please specify a css theme file."))));
     }
 
     @Override
     public void setValues() {
-        fontOverrideProperty.setValue(preferences.getBoolean(JabRefPreferences.OVERRIDE_DEFAULT_FONT_SIZE));
-        fontSizeProperty.setValue(String.valueOf(preferences.getInt(JabRefPreferences.MAIN_FONT_SIZE)));
-
-        switch (preferences.get(JabRefPreferences.FX_THEME)) {
-            case ThemeLoader.DARK_CSS:
-                themeLightProperty.setValue(false);
-                themeDarkProperty.setValue(true);
-                break;
-            case ThemeLoader.MAIN_CSS:
-            default:
-                themeLightProperty.setValue(true);
-                themeDarkProperty.setValue(false);
+        fontOverrideProperty.setValue(initialAppearancePreferences.shouldOverrideDefaultFontSize());
+        fontSizeProperty.setValue(String.valueOf(initialAppearancePreferences.getMainFontSize()));
+
+        Theme currentTheme = initialAppearancePreferences.getTheme();
+        if (currentTheme.getType() == Theme.Type.LIGHT) {
+            themeLightProperty.setValue(true);
+            themeDarkProperty.setValue(false);
+            themeCustomProperty.setValue(false);
+        } else if (currentTheme.getType() == Theme.Type.DARK) {
+            themeLightProperty.setValue(false);
+            themeDarkProperty.setValue(true);
+            themeCustomProperty.setValue(false);
+        } else {
+            themeLightProperty.setValue(false);
+            themeDarkProperty.setValue(false);
+            themeCustomProperty.setValue(true);
+            customPathToThemeProperty.setValue(currentTheme.getPath().toString());
         }
     }
 
     @Override
     public void storeSettings() {
-        if (preferences.getBoolean(JabRefPreferences.OVERRIDE_DEFAULT_FONT_SIZE) != fontOverrideProperty.getValue()) {
+        if (initialAppearancePreferences.shouldOverrideDefaultFontSize() != fontOverrideProperty.getValue()) {
             restartWarnings.add(Localization.lang("Override font settings"));
-            preferences.putBoolean(JabRefPreferences.OVERRIDE_DEFAULT_FONT_SIZE, fontOverrideProperty.getValue());
         }
 
         int newFontSize = Integer.parseInt(fontSizeProperty.getValue());
-        if (preferences.getInt(JabRefPreferences.MAIN_FONT_SIZE) != newFontSize) {
+        if (initialAppearancePreferences.getMainFontSize() != newFontSize) {
             restartWarnings.add(Localization.lang("Override font size"));
-            preferences.putInt(JabRefPreferences.MAIN_FONT_SIZE, newFontSize);
         }
 
-        if (themeLightProperty.getValue() && !preferences.get(JabRefPreferences.FX_THEME).equals(ThemeLoader.MAIN_CSS)) {
+        Theme newTheme = initialAppearancePreferences.getTheme();
+        if (themeLightProperty.getValue() && initialAppearancePreferences.getTheme().getType() != Theme.Type.LIGHT) {
             restartWarnings.add(Localization.lang("Theme changed to light theme."));
-            preferences.put(JabRefPreferences.FX_THEME, ThemeLoader.MAIN_CSS);
-        } else if (themeDarkProperty.getValue() && !preferences.get(JabRefPreferences.FX_THEME).equals(ThemeLoader.DARK_CSS)) {
+            newTheme = new Theme("", preferences);
+        } else if (themeDarkProperty.getValue() && initialAppearancePreferences.getTheme().getType() != Theme.Type.DARK) {
             restartWarnings.add(Localization.lang("Theme changed to dark theme."));
-            preferences.put(JabRefPreferences.FX_THEME, ThemeLoader.DARK_CSS);
+            newTheme = new Theme(EMBEDDED_DARK_THEME_CSS, preferences);
+        } else if (themeCustomProperty.getValue() &&
+                (!initialAppearancePreferences.getTheme().getPath().toString()
+                                              .equalsIgnoreCase(customPathToThemeProperty.getValue())
+                        || initialAppearancePreferences.getTheme().getType() != Theme.Type.CUSTOM)) {
+            restartWarnings.add(Localization.lang("Theme changed to a custom theme:") + " "
+                    + customPathToThemeProperty().getValue());
+            newTheme = new Theme(customPathToThemeProperty.getValue(), preferences);
         }
+
+        preferences.storeAppearancePreference(new AppearancePreferences(
+                fontOverrideProperty.getValue(),
+                newFontSize,
+                newTheme));
+
+        preferences.updateTheme();
     }
 
     public ValidationStatus fontSizeValidationStatus() {
         return fontSizeValidator.getValidationStatus();
     }
 
+    public ValidationStatus customPathToThemeValidationStatus() {
+        return customPathToThemeValidator.getValidationStatus();
+    }
+
     @Override
     public boolean validateSettings() {
-        if (fontOverrideProperty.getValue() && !fontSizeValidator.getValidationStatus().isValid()) {
-            fontSizeValidator.getValidationStatus().getHighestMessage().ifPresent(message ->
+        CompositeValidator validator = new CompositeValidator();
+
+        if (fontOverrideProperty.getValue()) {
+            validator.addValidators(fontSizeValidator);
+        }
+
+        if (themeCustomProperty.getValue()) {
+            validator.addValidators(customPathToThemeValidator);
+        }
+
+        ValidationStatus validationStatus = validator.getValidationStatus();
+        if (!validationStatus.isValid()) {
+            validationStatus.getHighestMessage().ifPresent(message ->
                     dialogService.showErrorDialogAndWait(message.getMessage()));
             return false;
         }
@@ -128,4 +181,22 @@ public BooleanProperty themeLightProperty() {
     public BooleanProperty themeDarkProperty() {
         return themeDarkProperty;
     }
+
+    public BooleanProperty customThemeProperty() {
+        return themeCustomProperty;
+    }
+
+    public StringProperty customPathToThemeProperty() {
+        return customPathToThemeProperty;
+    }
+
+    public void importCSSFile() {
+        FileDialogConfiguration fileDialogConfiguration = new FileDialogConfiguration.Builder()
+                .addExtensionFilter(StandardFileType.CSS)
+                .withDefaultExtension(StandardFileType.CSS)
+                .withInitialDirectory(preferences.setLastPreferencesExportPath()).build();
+
+        dialogService.showFileOpenDialog(fileDialogConfiguration).ifPresent(file ->
+                customPathToThemeProperty.setValue(file.toAbsolutePath().toString()));
+    }
 }
diff --git a/src/main/java/org/jabref/gui/preview/PreviewPanel.java b/src/main/java/org/jabref/gui/preview/PreviewPanel.java
index d9732ecc447..e0d5841857c 100644
--- a/src/main/java/org/jabref/gui/preview/PreviewPanel.java
+++ b/src/main/java/org/jabref/gui/preview/PreviewPanel.java
@@ -54,7 +54,7 @@ public PreviewPanel(BibDatabaseContext database, DialogService dialogService, Ex
         previewView = new PreviewViewer(database, dialogService, Globals.stateManager);
         previewView.setLayout(previewPreferences.getCurrentPreviewStyle());
         previewView.setContextMenu(createPopupMenu());
-        previewView.setTheme(this.preferences.get(JabRefPreferences.FX_THEME));
+        previewView.setTheme(this.preferences.getTheme());
         previewView.setOnDragDetected(event -> {
             previewView.startFullDrag();
 
diff --git a/src/main/java/org/jabref/gui/preview/PreviewViewer.java b/src/main/java/org/jabref/gui/preview/PreviewViewer.java
index ad950c7f078..79993768b29 100644
--- a/src/main/java/org/jabref/gui/preview/PreviewViewer.java
+++ b/src/main/java/org/jabref/gui/preview/PreviewViewer.java
@@ -22,7 +22,7 @@
 import org.jabref.gui.StateManager;
 import org.jabref.gui.util.BackgroundTask;
 import org.jabref.gui.util.TaskExecutor;
-import org.jabref.gui.util.ThemeLoader;
+import org.jabref.gui.util.Theme;
 import org.jabref.logic.exporter.ExporterFactory;
 import org.jabref.logic.l10n.Localization;
 import org.jabref.logic.preview.PreviewLayout;
@@ -116,11 +116,11 @@ public PreviewViewer(BibDatabaseContext database, DialogService dialogService, S
         });
     }
 
-    public void setTheme(String theme) {
-        if (theme.equals(ThemeLoader.DARK_CSS)) {
+    public void setTheme(Theme theme) {
+        if (theme.getType() == Theme.Type.DARK) {
             // We need to load the css file manually, due to a bug in the jdk
             // TODO: Remove this workaround as soon as https://github.com/openjdk/jfx/pull/22 is merged
-            URL url = JabRefFrame.class.getResource(ThemeLoader.DARK_CSS);
+            URL url = JabRefFrame.class.getResource(theme.getPath().getFileName().toString());
             String dataUrl = "data:text/css;charset=utf-8;base64," +
                     Base64.getEncoder().encodeToString(StringUtil.getResourceFileAsString(url).getBytes());
 
diff --git a/src/main/java/org/jabref/gui/util/BaseDialog.java b/src/main/java/org/jabref/gui/util/BaseDialog.java
index 921be10e748..1995d34ce7d 100644
--- a/src/main/java/org/jabref/gui/util/BaseDialog.java
+++ b/src/main/java/org/jabref/gui/util/BaseDialog.java
@@ -36,7 +36,7 @@ protected BaseDialog() {
 
         setDialogIcon(IconTheme.getJabRefImageFX());
         setResizable(true);
-        Globals.getThemeLoader().installCss(getDialogPane().getScene(), Globals.prefs);
+        Globals.prefs.getTheme().installCss(getDialogPane().getScene());
     }
 
     private Optional<Button> getDefaultButton() {
diff --git a/src/main/java/org/jabref/gui/util/Theme.java b/src/main/java/org/jabref/gui/util/Theme.java
new file mode 100644
index 00000000000..dd4788b58d7
--- /dev/null
+++ b/src/main/java/org/jabref/gui/util/Theme.java
@@ -0,0 +1,137 @@
+package org.jabref.gui.util;
+
+import java.io.IOException;
+import java.net.MalformedURLException;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Optional;
+
+import javafx.scene.Scene;
+
+import org.jabref.Globals;
+import org.jabref.gui.JabRefFrame;
+import org.jabref.model.strings.StringUtil;
+import org.jabref.model.util.FileUpdateMonitor;
+import org.jabref.preferences.AppearancePreferences;
+import org.jabref.preferences.PreferencesService;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Installs the style file and provides live reloading.
+ * JabRef provides two inbuilt themes and a user customizable one: Light, Dark and Custom. The Light theme is basically
+ * the base.css theme. Every other theme is loaded as an addition to base.css.
+ */
+public class Theme {
+    public enum Type {
+        LIGHT, DARK, CUSTOM
+    }
+
+    public static final String BASE_CSS = "Base.css";
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(Theme.class);
+
+    private final Type type;
+    private final Path pathToCss;
+    private final Optional<URL> additionalCssToLoad;
+    private final PreferencesService preferencesService;
+
+    public Theme(String path, PreferencesService preferencesService) {
+        this.pathToCss = Path.of(path);
+        this.preferencesService = preferencesService;
+
+        if (StringUtil.isBlank(path) || BASE_CSS.equalsIgnoreCase(path)) {
+            // Light theme
+            this.type = Type.LIGHT;
+            this.additionalCssToLoad = Optional.empty();
+        } else {
+            Optional<URL> cssResource = Optional.ofNullable(JabRefFrame.class.getResource(path));
+            if (cssResource.isPresent()) {
+                // Embedded dark theme
+                this.type = Type.DARK;
+            } else {
+                // Custom theme
+                this.type = Type.CUSTOM;
+                if (Files.exists(pathToCss)) {
+                    try {
+                        cssResource = Optional.of(pathToCss.toUri().toURL());
+                    } catch (MalformedURLException e) {
+                        cssResource = Optional.empty();
+                    }
+                }
+            }
+
+            if (cssResource.isPresent()) {
+                additionalCssToLoad = cssResource;
+                LOGGER.debug("Using css {}", path);
+            } else {
+                additionalCssToLoad = Optional.empty();
+                LOGGER.warn("Cannot load css {}", path);
+            }
+        }
+    }
+
+    /**
+     * Installs the base css file as a stylesheet in the given scene. Changes in the css file lead to a redraw of the
+     * scene using the new css file.
+     */
+    public void installCss(Scene scene, FileUpdateMonitor fileUpdateMonitor) {
+        AppearancePreferences appearancePreferences = preferencesService.getAppearancePreferences();
+
+        addAndWatchForChanges(scene, JabRefFrame.class.getResource(BASE_CSS), fileUpdateMonitor, 0);
+        additionalCssToLoad.ifPresent(file -> addAndWatchForChanges(scene, file, fileUpdateMonitor, 1));
+
+        if (appearancePreferences.shouldOverrideDefaultFontSize()) {
+            scene.getRoot().setStyle("-fx-font-size: " + appearancePreferences.getMainFontSize() + "pt;");
+        }
+    }
+
+    /**
+     * StyleTester does not create a Globals object, so we need to wrap this method so the style tester can provide
+     * its own fileUpdateMonitor, but the main codebase is not spammed.
+     *
+     * @deprecated Remove as soon {@link org.jabref.Globals} refactored.
+     *
+     * @param scene the scene the css should be applied to.
+     */
+    @Deprecated
+    public void installCss(Scene scene) {
+        installCss(scene, Globals.getFileUpdateMonitor());
+    }
+
+    private void addAndWatchForChanges(Scene scene, URL cssFile, FileUpdateMonitor fileUpdateMonitor, int index) {
+        scene.getStylesheets().add(index, cssFile.toExternalForm());
+
+        try {
+            // If the file is an ordinary file (i.e. not part of a java runtime bundle), we watch it for changes and turn on live reloading
+            URI cssUri = cssFile.toURI();
+            if (!cssUri.toString().contains("jrt")) {
+                LOGGER.debug("CSS URI {}", cssUri);
+
+                Path cssPath = Path.of(cssUri).toAbsolutePath();
+                LOGGER.info("Enabling live reloading of {}", cssPath);
+                fileUpdateMonitor.addListenerForFile(cssPath, () -> {
+                    LOGGER.info("Reload css file {}", cssFile);
+                    DefaultTaskExecutor.runInJavaFXThread(() -> {
+                        scene.getStylesheets().remove(cssFile.toExternalForm());
+                        scene.getStylesheets().add(index, cssFile.toExternalForm());
+                    });
+                });
+            }
+        } catch (IOException | URISyntaxException | UnsupportedOperationException e) {
+            LOGGER.error("Could not watch css file for changes {}", cssFile, e);
+        }
+    }
+
+    public Type getType() {
+        return type;
+    }
+
+    public Path getPath() {
+        return pathToCss;
+    }
+}
diff --git a/src/main/java/org/jabref/gui/util/ThemeLoader.java b/src/main/java/org/jabref/gui/util/ThemeLoader.java
deleted file mode 100644
index 22cd37a4d28..00000000000
--- a/src/main/java/org/jabref/gui/util/ThemeLoader.java
+++ /dev/null
@@ -1,109 +0,0 @@
-package org.jabref.gui.util;
-
-import java.io.IOException;
-import java.net.MalformedURLException;
-import java.net.URI;
-import java.net.URISyntaxException;
-import java.net.URL;
-import java.nio.file.Path;
-import java.util.Objects;
-import java.util.Optional;
-
-import javafx.scene.Scene;
-
-import org.jabref.gui.JabRefFrame;
-import org.jabref.model.strings.StringUtil;
-import org.jabref.model.util.FileUpdateMonitor;
-import org.jabref.preferences.JabRefPreferences;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
-/**
- * Installs the style file and provides live reloading.
- * <p>
- * The live reloading has to be turned on by setting the <code>-Djabref.theme.css</code> property.
- * There two possible modes:
- * (1) When only <code>-Djabref.theme.css</code> is specified, then the standard <code>Base.css</code> that is found will be watched
- * and on changes in that file, the style-sheet will be reloaded and changes are immediately visible.
- * (2) When a path to a css file is passed to <code>-Djabref.theme.css</code>, then the given style is loaded in addition to the base css file.
- * Changes in the specified css file lead to an immediate redraw of the interface.
- * <p>
- * When working from an IDE, this usually means that the <code>Base.css</code> is located in the build folder.
- * To use the css-file that is located in the sources directly, the full path can be given as value for the "VM option":
- * <code>-Djabref.theme.css="/path/to/src/Base.css"</code>
- */
-public class ThemeLoader {
-
-    public static final String MAIN_CSS = "Base.css";
-    public static final String DARK_CSS = "Dark.css";
-
-    private static final Logger LOGGER = LoggerFactory.getLogger(ThemeLoader.class);
-    private final Optional<URL> additionalCssToLoad;
-    private final FileUpdateMonitor fileUpdateMonitor;
-
-    public ThemeLoader(FileUpdateMonitor fileUpdateMonitor, JabRefPreferences jabRefPreferences) {
-        this.fileUpdateMonitor = Objects.requireNonNull(fileUpdateMonitor);
-
-        String cssVmArgument = System.getProperty("jabref.theme.css");
-        String cssPreferences = jabRefPreferences.get(JabRefPreferences.FX_THEME);
-        if (StringUtil.isNotBlank(cssVmArgument)) {
-            // First priority: VM argument
-            LOGGER.info("Using css from VM option: " + cssVmArgument);
-            URL cssVmUrl = null;
-            try {
-                cssVmUrl = Path.of(cssVmArgument).toUri().toURL();
-            } catch (MalformedURLException e) {
-                LOGGER.warn("Cannot load css " + cssVmArgument, e);
-            }
-            additionalCssToLoad = Optional.ofNullable(cssVmUrl);
-        } else if (StringUtil.isNotBlank(cssPreferences) && !MAIN_CSS.equalsIgnoreCase(cssPreferences)) {
-            // Otherwise load css from preference
-            URL cssResource = JabRefFrame.class.getResource(cssPreferences);
-            if (cssResource != null) {
-                LOGGER.debug("Using css " + cssResource);
-                additionalCssToLoad = Optional.of(cssResource);
-            } else {
-                additionalCssToLoad = Optional.empty();
-                LOGGER.warn("Cannot load css " + cssPreferences);
-            }
-        } else {
-            additionalCssToLoad = Optional.empty();
-        }
-    }
-
-    /**
-     * Installs the base css file as a stylesheet in the given scene. Changes in the css file lead to a redraw of the
-     * scene using the new css file.
-     */
-    public void installCss(Scene scene, JabRefPreferences preferences) {
-        addAndWatchForChanges(scene, JabRefFrame.class.getResource(MAIN_CSS), 0);
-        additionalCssToLoad.ifPresent(file -> addAndWatchForChanges(scene, file, 1));
-
-        preferences.getFontSize().ifPresent(size -> scene.getRoot().setStyle("-fx-font-size: " + size + "pt;"));
-    }
-
-    private void addAndWatchForChanges(Scene scene, URL cssFile, int index) {
-        scene.getStylesheets().add(index, cssFile.toExternalForm());
-
-        try {
-            // If the file is an ordinary file (i.e. not part of a java runtime bundle), we watch it for changes and turn on live reloading
-            URI cssUri = cssFile.toURI();
-            if (!cssUri.toString().contains("jrt")) {
-                LOGGER.debug("CSS URI {}", cssUri);
-
-                Path cssPath = Path.of(cssUri).toAbsolutePath();
-                LOGGER.info("Enabling live reloading of {}", cssPath);
-                fileUpdateMonitor.addListenerForFile(cssPath, () -> {
-                    LOGGER.info("Reload css file {}", cssFile);
-                    DefaultTaskExecutor.runInJavaFXThread(() -> {
-                        scene.getStylesheets().remove(cssFile.toExternalForm());
-                        scene.getStylesheets().add(index, cssFile.toExternalForm());
-                    });
-                });
-            }
-        } catch (IOException | URISyntaxException | UnsupportedOperationException e) {
-            LOGGER.error("Could not watch css file for changes " + cssFile, e);
-        }
-    }
-}
diff --git a/src/main/java/org/jabref/logic/util/StandardFileType.java b/src/main/java/org/jabref/logic/util/StandardFileType.java
index b186d9a39e9..45c48b6cf76 100644
--- a/src/main/java/org/jabref/logic/util/StandardFileType.java
+++ b/src/main/java/org/jabref/logic/util/StandardFileType.java
@@ -40,7 +40,8 @@ public enum StandardFileType implements FileType {
     XML("xml"),
     JSON("json"),
     XMP("xmp"),
-    ZIP("zip");
+    ZIP("zip"),
+    CSS("css");
 
     private final List<String> extensions;
 
diff --git a/src/main/java/org/jabref/preferences/AppearancePreferences.java b/src/main/java/org/jabref/preferences/AppearancePreferences.java
new file mode 100644
index 00000000000..09b399536c6
--- /dev/null
+++ b/src/main/java/org/jabref/preferences/AppearancePreferences.java
@@ -0,0 +1,27 @@
+package org.jabref.preferences;
+
+import org.jabref.gui.util.Theme;
+
+public class AppearancePreferences {
+    private final boolean shouldOverrideDefaultFontSize;
+    private final int mainFontSize;
+    private final Theme theme;
+
+    public AppearancePreferences(boolean shouldOverrideDefaultFontSize, int mainFontSize, Theme theme) {
+        this.shouldOverrideDefaultFontSize = shouldOverrideDefaultFontSize;
+        this.mainFontSize = mainFontSize;
+        this.theme = theme;
+    }
+
+    public boolean shouldOverrideDefaultFontSize() {
+        return shouldOverrideDefaultFontSize;
+    }
+
+    public int getMainFontSize() {
+        return mainFontSize;
+    }
+
+    public Theme getTheme() {
+        return theme;
+    }
+}
diff --git a/src/main/java/org/jabref/preferences/JabRefPreferences.java b/src/main/java/org/jabref/preferences/JabRefPreferences.java
index ffacd6968ea..b07ba7fe889 100644
--- a/src/main/java/org/jabref/preferences/JabRefPreferences.java
+++ b/src/main/java/org/jabref/preferences/JabRefPreferences.java
@@ -55,7 +55,7 @@
 import org.jabref.gui.preferences.ImportTabViewModel;
 import org.jabref.gui.search.SearchDisplayMode;
 import org.jabref.gui.specialfields.SpecialFieldsPreferences;
-import org.jabref.gui.util.ThemeLoader;
+import org.jabref.gui.util.Theme;
 import org.jabref.logic.bibtex.FieldContentFormatterPreferences;
 import org.jabref.logic.bibtex.FieldWriterPreferences;
 import org.jabref.logic.citationkeypattern.CitationKeyPatternPreferences;
@@ -414,6 +414,11 @@ public class JabRefPreferences implements PreferencesService {
      */
     private List<MainTableColumnModel> mainTableColumnSortOrder;
 
+    /**
+     * Cache variable for getTheme
+     */
+    private Theme globalTheme;
+
     // The constructor is made private to enforce this as a singleton class:
     private JabRefPreferences() {
         try {
@@ -703,7 +708,7 @@ private JabRefPreferences() {
                         + "</dd>__NEWLINE__<p></p></font>");
 
         // set default theme
-        defaults.put(JabRefPreferences.FX_THEME, ThemeLoader.MAIN_CSS);
+        defaults.put(JabRefPreferences.FX_THEME, Theme.BASE_CSS);
 
         setLanguageDependentDefaultValues();
     }
@@ -819,11 +824,6 @@ public String getUser() {
         }
     }
 
-    @Override
-    public String getTheme() {
-        return get(FX_THEME);
-    }
-
     public void setLanguageDependentDefaultValues() {
         // Entry editor tab 0:
         defaults.put(CUSTOM_TAB_NAME + "_def0", Localization.lang("General"));
@@ -1125,7 +1125,7 @@ public void exportPreferences(String filename) throws JabRefException {
     }
 
     public void exportPreferences(Path file) throws JabRefException {
-        LOGGER.debug("Exporting preferences ", file.toAbsolutePath());
+        LOGGER.debug("Exporting preferences {}", file.toAbsolutePath());
         try (OutputStream os = Files.newOutputStream(file)) {
             prefs.exportSubtree(os);
         } catch (BackingStoreException | IOException ex) {
@@ -1564,14 +1564,7 @@ public String getPreviewStyle() {
         return get(PREVIEW_STYLE);
     }
 
-    public Optional<Integer> getFontSize() {
-        if (getBoolean(OVERRIDE_DEFAULT_FONT_SIZE)) {
-            return Optional.of(getInt(MAIN_FONT_SIZE));
-        } else {
-            return Optional.empty();
-        }
-    }
-
+    @Override
     public String setLastPreferencesExportPath() {
         return get(PREFS_EXPORT_PATH);
     }
@@ -2319,6 +2312,38 @@ public void storeMainTableNameFormatPreferences(MainTableNameFormatPreferences p
         putBoolean(JabRefPreferences.NAMES_LAST_ONLY, preferences.getAbbreviationStyle() == AbbreviationStyle.LASTNAME_ONLY);
     }
 
+    //*************************************************************************************************************
+    // AppearancePreferences
+    //*************************************************************************************************************
+
+    @Override
+    public Theme getTheme() {
+        if (globalTheme == null) {
+            updateTheme();
+        }
+        return globalTheme;
+    }
+
+    @Override
+    public void updateTheme() {
+        this.globalTheme = new Theme(get(FX_THEME), this);
+    }
+
+    @Override
+    public AppearancePreferences getAppearancePreferences() {
+        return new AppearancePreferences(
+                getBoolean(OVERRIDE_DEFAULT_FONT_SIZE),
+                getInt(MAIN_FONT_SIZE),
+                getTheme());
+    }
+
+    @Override
+    public void storeAppearancePreference(AppearancePreferences preferences) {
+        putBoolean(OVERRIDE_DEFAULT_FONT_SIZE, preferences.shouldOverrideDefaultFontSize());
+        putInt(MAIN_FONT_SIZE, preferences.getMainFontSize());
+        put(FX_THEME, preferences.getTheme().getPath().toString());
+    }
+
     //*************************************************************************************************************
     // ToDo: Misc preferences
     //*************************************************************************************************************
diff --git a/src/main/java/org/jabref/preferences/PreferencesService.java b/src/main/java/org/jabref/preferences/PreferencesService.java
index 410dab570c2..f521b0a7315 100644
--- a/src/main/java/org/jabref/preferences/PreferencesService.java
+++ b/src/main/java/org/jabref/preferences/PreferencesService.java
@@ -14,6 +14,7 @@
 import org.jabref.gui.maintable.MainTableNameFormatPreferences;
 import org.jabref.gui.maintable.MainTablePreferences;
 import org.jabref.gui.specialfields.SpecialFieldsPreferences;
+import org.jabref.gui.util.Theme;
 import org.jabref.logic.bibtex.FieldContentFormatterPreferences;
 import org.jabref.logic.bibtex.FieldWriterPreferences;
 import org.jabref.logic.citationkeypattern.CitationKeyPatternPreferences;
@@ -72,6 +73,8 @@ public interface PreferencesService {
 
     void setWorkingDir(Path dir);
 
+    String setLastPreferencesExportPath();
+
     OpenOfficePreferences getOpenOfficePreferences();
 
     void setOpenOfficePreferences(OpenOfficePreferences openOfficePreferences);
@@ -98,8 +101,6 @@ public interface PreferencesService {
 
     String getUser();
 
-    String getTheme();
-
     SaveOrderConfig loadExportSaveOrder();
 
     void storeExportSaveOrder(SaveOrderConfig config);
@@ -230,6 +231,18 @@ public interface PreferencesService {
 
     void storeMainTableNameFormatPreferences(MainTableNameFormatPreferences preferences);
 
+    //*************************************************************************************************************
+    // AppearancePreferences
+    //*************************************************************************************************************
+
+    Theme getTheme();
+
+    void updateTheme();
+
+    AppearancePreferences getAppearancePreferences();
+
+    void storeAppearancePreference(AppearancePreferences preferences);
+
     //*************************************************************************************************************
     // ToDo: Misc preferences
     //*************************************************************************************************************
diff --git a/src/main/java/org/jabref/styletester/StyleTesterMain.java b/src/main/java/org/jabref/styletester/StyleTesterMain.java
index 13fa1ffcf7c..8d1b605c6b7 100644
--- a/src/main/java/org/jabref/styletester/StyleTesterMain.java
+++ b/src/main/java/org/jabref/styletester/StyleTesterMain.java
@@ -8,7 +8,6 @@
 import org.jabref.JabRefExecutorService;
 import org.jabref.gui.icon.IconTheme;
 import org.jabref.gui.util.DefaultFileUpdateMonitor;
-import org.jabref.gui.util.ThemeLoader;
 import org.jabref.preferences.JabRefPreferences;
 
 /**
@@ -23,6 +22,7 @@ public static void main(String[] args) {
     @Override
     public void start(Stage stage) throws JabRefException {
         StyleTesterView view = new StyleTesterView();
+        JabRefPreferences preferences = JabRefPreferences.getInstance();
 
         IconTheme.loadFonts();
 
@@ -30,7 +30,7 @@ public void start(Stage stage) throws JabRefException {
         JabRefExecutorService.INSTANCE.executeInterruptableTask(fileUpdateMonitor, "FileUpdateMonitor");
 
         Scene scene = new Scene(view.getContent());
-        new ThemeLoader(fileUpdateMonitor, JabRefPreferences.getInstance()).installCss(scene, JabRefPreferences.getInstance());
+        preferences.getTheme().installCss(scene, fileUpdateMonitor);
         stage.setScene(scene);
         stage.show();
     }
diff --git a/src/main/resources/l10n/JabRef_en.properties b/src/main/resources/l10n/JabRef_en.properties
index 6f41d5675c0..15954e36a76 100644
--- a/src/main/resources/l10n/JabRef_en.properties
+++ b/src/main/resources/l10n/JabRef_en.properties
@@ -2020,6 +2020,7 @@ Font=Font
 Visual\ theme=Visual theme
 Light\ theme=Light theme
 Dark\ theme=Dark theme
+Custom\ theme=Custom theme
 Overwrite\ existing\ keys=Overwrite existing keys
 Key\ patterns=Key patterns
 Font\ settings=Font settings
@@ -2027,6 +2028,8 @@ Override\ font\ settings=Override font settings
 Override\ font\ size=Override font size
 Theme\ changed\ to\ dark\ theme.=Theme changed to dark theme.
 Theme\ changed\ to\ light\ theme.=Theme changed to light theme.
+Theme\ changed\ to\ a\ custom\ theme\:=Theme changed to a custom theme:
+Please\ specify\ a\ css\ theme\ file.=Please specify a css theme file.
 You\ must\ enter\ an\ integer\ value\ higher\ than\ 8.=You must enter an integer value higher than 8.
 Letters\ after\ duplicate\ generated\ keys=Letters after duplicate generated keys
 Start\ on\ second\ duplicate\ key\ with\ letter\ A\ (a,\ b,\ ...)=Start on second duplicate key with letter A (a, b, ...)