[GAR][a11y] Update assessment app to unblock tester from testing dark mode (#149055)

internal: b/317130300

reopening #149004

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dev/a11y_assessments/lib/main.dart

@@ -26,10 +26,8 @@ class App extends StatelessWidget {
     );
     return MaterialApp(
       title: 'Accessibility Assessments',
-      theme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
-        useMaterial3: true,
-      ),
+      theme: ThemeData.light(),
+      darkTheme: ThemeData.dark(),
       routes: <String, WidgetBuilder>{
         '/': (_) => const HomePage(),
         ...routes

dev/a11y_assessments/test/home_page_test.dart

@@ -0,0 +1,17 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:a11y_assessments/main.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+
+void main() {
+  testWidgets('Has light and dark theme', (WidgetTester tester) async {
+    await tester.pumpWidget(const App());
+    final MaterialApp app = find.byType(MaterialApp).evaluate().first.widget as MaterialApp;
+    expect(app.theme!.brightness, equals(Brightness.light));
+    expect(app.darkTheme!.brightness, equals(Brightness.dark));
+  });
+}