refactor: update styling and color classes across components

- Changed error message styling in ICalImportModal to use new color classes.
- Updated task completion styling in TaskListView to use new success color classes.
- Modified selected styling in ContactListItem tests to reflect new background class.
- Adjusted duplicate warning styling in ContactImportDialog to use new warning color classes.
- Refactored background and border colors in CalendarInvitationBanner for various statuses.
- Updated email list item and viewer components to use new warning and success color classes.
- Refined styling for unread indicators in email components.
- Enhanced error fallback styling in error components to use new warning color classes.
- Updated filter rule modal button hover styles to use new destructive color classes.
- Added experimental feature descriptions in Plugins and Themes settings.
- Refined vacation settings validation warning styling to use new warning color classes.
- Updated toast component styles to use new color classes for different states.
- Introduced a new built-in theme 'Qui' with specific color variables.
- Adjusted email security status colors to use new warning color classes.

components/settings/themes-settings.tsx

@@ -69,7 +69,8 @@ export function ThemesSettings() {
   };
 
   return (
-    <SettingsSection title="Themes" description="Customize the appearance with color themes. Upload .zip theme files or activate built-in presets.">
+    <SettingsSection title="Themes" description="Customize the appearance with color themes. Upload .zip theme files or activate built-in presets." experimental experimentalDescription="Themes is an experimental feature. Custom themes may not cover all UI elements, and theme formats could change in future updates. Built-in presets are stable, but uploaded themes may require updates after application upgrades.">
+
       {/* Theme Grid */}
       <div className="grid grid-cols-2 sm:grid-cols-3 gap-3">
         {/* Default theme card */}