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

packages/z-timeline/lib/state.dart

@@ -11,6 +11,7 @@ class TimelineState {
   final Map<String, TimelineItemData> items;
   final List<String> groupOrder;
   final String? selectedItemId;
+  final bool darkMode;
 
   TimelineState({
     required this.timeline,
@@ -18,6 +19,7 @@ class TimelineState {
     required this.items,
     required this.groupOrder,
     this.selectedItemId,
+    this.darkMode = true,
   });
 
   factory TimelineState.fromJson(Map<String, dynamic> json) {
@@ -36,6 +38,7 @@ class TimelineState {
       ),
       groupOrder: (json['groupOrder'] as List<dynamic>).cast<String>(),
       selectedItemId: json['selectedItemId'] as String?,
+      darkMode: json['darkMode'] as bool? ?? true,
     );
   }
 }