feat(ground-suite): add theme selector

ground-suite/lib/app/theme_mode_controller.dart

@@ -0,0 +1,17 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+final themeModeProvider = NotifierProvider<ThemeModeController, ThemeMode>(
+  ThemeModeController.new,
+);
+
+/// Holds the local display preference for the current application session.
+///
+/// It intentionally starts at the system preference. Persisting an operator's
+/// visual preference can be added later without coupling it to vehicle state.
+class ThemeModeController extends Notifier<ThemeMode> {
+  @override
+  ThemeMode build() => ThemeMode.system;
+
+  void select(ThemeMode mode) => state = mode;
+}