add dark theme toggle

packages/z-timeline/lib/main.dart

@@ -20,6 +20,7 @@ class _MainAppState extends State<MainApp> {
   List<TimelineGroup> _groups = const [];
   List<TimelineEntry> _entries = const [];
   TimelineViewportNotifier? _viewport;
+  bool _darkMode = true;
 
   @override
   void initState() {
@@ -53,6 +54,7 @@ class _MainAppState extends State<MainApp> {
       _state = state;
       _groups = groups;
       _entries = entries;
+      _darkMode = state.darkMode;
 
       _viewport ??= TimelineViewportNotifier(
         start: domain.start,
@@ -207,7 +209,9 @@ class _MainAppState extends State<MainApp> {
 
     return MaterialApp(
       debugShowCheckedModeBanner: false,
-      theme: ThemeData.dark(useMaterial3: true),
+      theme: _darkMode
+          ? ThemeData.dark(useMaterial3: true)
+          : ThemeData.light(useMaterial3: true),
       home: Scaffold(
         backgroundColor: Colors.transparent,
         body: _state == null || viewport == null