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/email/thread-list-item.tsx

@@ -156,7 +156,7 @@ const SingleEmailItem = React.forwardRef<HTMLDivElement, SingleEmailItemProps>(
 
           {isUnread && (
             <div className="absolute left-1 top-1/2 -translate-y-1/2">
-              <Circle className="w-2 h-2 fill-blue-600 text-blue-600 dark:fill-blue-400 dark:text-blue-400" />
+              <Circle className="w-2 h-2 fill-unread text-unread" />
             </div>
           )}
 
@@ -456,7 +456,7 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
 
             {hasUnread && (
               <div className="absolute left-1 top-1/2 -translate-y-1/2">
-                <Circle className="w-2 h-2 fill-blue-600 text-blue-600 dark:fill-blue-400 dark:text-blue-400" />
+                <Circle className="w-2 h-2 fill-unread text-unread" />
               </div>
             )}