UI/Qt: Use Qt-reported dark theme

For Qt >= 6.5, the system theme can be determined reliably, so no
guesswork is needed. A fallback remains for Qt < 6.5, but it is
hacky and less reliable.

UI/Qt/main.cpp

@@ -22,6 +22,10 @@
 #include <UI/Qt/Settings.h>
 #include <UI/Qt/WebContentView.h>
 
+#if QT_VERSION >= QT_VERSION_CHECK(6, 5, 0)
+#    include <QStyleHints>
+#endif
+
 #if defined(AK_OS_MACOS)
 #    include <LibWebView/MachPortServer.h>
 #endif
@@ -32,15 +36,20 @@ namespace Ladybird {
 bool is_using_dark_system_theme(QWidget&);
 bool is_using_dark_system_theme(QWidget& widget)
 {
-    // FIXME: Qt does not provide any method to query if the system is using a dark theme. We will have to implement
-    //        platform-specific methods if we wish to have better detection. For now, this inspects if Qt is using a
-    //        dark color for widget backgrounds using Rec. 709 luma coefficients.
-    //        https://en.wikipedia.org/wiki/Rec._709#Luma_coefficients
-
+#if QT_VERSION >= QT_VERSION_CHECK(6, 5, 0)
+    // We use that only for fallback mode
+    Q_UNUSED(widget);
+    // Use the new Qt API available from version 6.5.0
+    auto color_scheme = QGuiApplication::styleHints()->colorScheme();
+    return color_scheme == Qt::ColorScheme::Dark;
+#else
+    // Fallback for older Qt versions
+    // Calculate luma based on Rec. 709 coefficients
+    // https://en.wikipedia.org/wiki/Rec._709#Luma_coefficients
     auto color = widget.palette().color(widget.backgroundRole());
     auto luma = 0.2126f * color.redF() + 0.7152f * color.greenF() + 0.0722f * color.blueF();
-
     return luma <= 0.5f;
+#endif
 }
 
 }