Migrate mui themes to css vars

src/hooks/useUserTheme.ts

@@ -1,12 +1,14 @@
 import { useThemes } from './useThemes';
 import { useUserSettings } from './useUserSettings';
 
+const FALLBACK_THEME_ID = 'dark';
+
 export function useUserTheme() {
     const { theme, dashboardTheme } = useUserSettings();
     const { defaultTheme } = useThemes();
 
     return {
-        theme: theme || defaultTheme?.id,
-        dashboardTheme: dashboardTheme || defaultTheme?.id
+        theme: theme || defaultTheme?.id || FALLBACK_THEME_ID,
+        dashboardTheme: dashboardTheme || defaultTheme?.id || FALLBACK_THEME_ID
     };
 }