riverpod_theme_app: service layer providers.

theming_service_2.dart

// Creates a themingServiceProvider used by the ThemingController.
@riverpod
ThemingService themingService(ThemingServiceRef ref) {
  final prefs = ref.read(prefsProvider);
  return ThemingService(prefs);
}

// Override in `Provider Container` when app starts.
@riverpod
SharedPreferences prefs(PrefsRef ref) => throw UnimplementedError();