add theme variables documentation (#823)

docs/customizations/index.md

@@ -5,6 +5,7 @@ The GeoNode MapStore Client application can be customized with the following app
 
 - [Change variables in settings.py](settings-variables.md)
 - [Edit localConfig.json via template override](override-local-config.md)
+- [Custom theme colors](theme-variables.md)
 
 Advanced customizations:
 - [Install a custom branch of the geonode-mapstore-client in the geonode-project](custom-branch.md)

docs/customizations/theme-variables.md

@@ -0,0 +1,25 @@
+# Custom theme colors
+
+The GeoNode theme allows to override colors with [css variables](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) in this ways the layout CSS structure will be always aligned with the version in use but with a customized look.
+
+Here the steps to overrides the variables:
+
+- add a style tag in the template. it is possible to use the `geonode-mapstore-client/snippets/custom_theme.html` template included in all the pages.
+
+```html
+<style>
+</style>
+```
+
+- add the `.gn-theme` class in the style tag with the css variable to override
+
+```html
+<style>
+    /* msgapi is the selector wrapper of the page */
+    .msgapi .gn-theme {
+        --gn-primary: #74cfe2;
+    }
+</style>
+```
+
+It is possible to use this [page/tool](theme.html) to customize and get the style snippet to apply to the theme.