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/vacation-settings.tsx

@@ -240,7 +240,7 @@ export function VacationSettings() {
       {validationWarnings.length > 0 && (
         <div className="space-y-2">
           {validationWarnings.map((warning, i) => (
-            <div key={i} className="flex items-start gap-2 text-sm text-amber-700 dark:text-amber-400">
+            <div key={i} className="flex items-start gap-2 text-sm text-warning">
               <AlertTriangle className="w-4 h-4 mt-0.5 flex-shrink-0" />
               <span>{warning}</span>
             </div>