Add refetchThemes function to event theme context and usage

Introduce a new `refetchThemes` function in the event theme context to allow for refreshing event theme data. Update `EventThemeForm` to call `refetchThemes` after updating a theme, ensuring the data stays synchronized. This enhances the consistency of the UI after changes to event themes.

frontend/src/components/event-themes/event-theme-form.tsx

@@ -36,7 +36,7 @@ type ThemeFormProps = {
 
 export function EventThemeForm({ theme, mode }: ThemeFormProps) {
   const router = useRouter();
-  const { createTheme, updateTheme } = useEventThemes();
+  const { createTheme, updateTheme, refetchThemes } = useEventThemes();
 
   // Form state
   const [formState, setFormState] = useState<
@@ -202,6 +202,7 @@ export function EventThemeForm({ theme, mode }: ThemeFormProps) {
         router.push("/dashboard/event-themes");
       } else if (mode === "edit" && theme) {
         await updateTheme(theme.id, formState as EventThemeUpdate);
+        await refetchThemes();
         router.push("/dashboard/event-themes");
       }
     } catch (err) {