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/calendar/ical-import-modal.tsx

@@ -349,7 +349,7 @@ export function ICalImportModal({ calendars, client, onClose }: ICalImportModalP
           )}
 
           {error && (
-            <div className="text-sm text-red-600 dark:text-red-400 bg-red-50 dark:bg-red-950/30 rounded-md px-3 py-2">
+            <div className="text-sm text-destructive bg-destructive/10 rounded-md px-3 py-2">
               {error}
             </div>
           )}