feat:增加主题颜色和暗黑模式

lib/widgets/todo_widget.dart

@@ -13,7 +13,7 @@ Widget buildStatsCard(List<Todo> todos) {
 
   int completedCount = todos.where((todo) => todo.isCompleted).length;
 
-  return buildCard(
+  return BuildCard(
     child: Row(
       mainAxisAlignment: MainAxisAlignment.spaceAround,
       children: [
@@ -44,6 +44,7 @@ Widget _buildStatItem(String label, int count, Color color) {
 
 // Tab页
 Widget buildTabs({
+  required BuildContext context,
   required TodoTab currentTab,
   required ValueChanged<TodoTab> onTabChanged,
 }) {
@@ -55,7 +56,7 @@ Widget buildTabs({
       initialLabelIndex: TodoTab.values.indexOf(currentTab),
       totalSwitches: TodoTab.values.length,
       labels: TodoTab.values.map((e) => e.label).toList(),
-      activeBgColor: [Colors.orange.shade600],
+      activeBgColor: [Theme.of(context).colorScheme.primary],
       activeFgColor: Colors.white,
       inactiveBgColor: Colors.grey.shade200,
       inactiveFgColor: Colors.grey.shade700,
@@ -94,7 +95,7 @@ Widget _buildTodoItem({
   required ValueChanged<Todo> onToggle,
   required ValueChanged<Todo> onEdit,
 }) {
-  return buildCard(
+  return BuildCard(
     child: ListTile(
       contentPadding: EdgeInsets.symmetric(horizontal: 8, vertical: 0),
       leading: SizedBox(
@@ -267,7 +268,7 @@ Widget _buildPriorityBadge(TodoPriority priority) {
 }
 
 // 空状态
-Widget buildEmptyState(TodoTab tab) {
+Widget buildEmptyState(BuildContext context, TodoTab tab) {
   final messages = {
     TodoTab.all: '📝 还没有待办事项\n点击➕号添加第一个任务吧～',
     TodoTab.active: '🎯 没有待完成的任务\n享受轻松时光吧！✨',
@@ -275,15 +276,18 @@ Widget buildEmptyState(TodoTab tab) {
     TodoTab.today: '📅 今天没有安排任务\n好好放松一下吧～😊',
   };
 
+  final colors = Theme.of(context).colorScheme;
+
   return Center(
     child: Column(
       mainAxisAlignment: MainAxisAlignment.center,
       children: [
-        Icon(Icons.checklist, size: 80, color: Colors.orange.shade600),
+        Icon(Icons.checklist, size: 80, color: colors.primary),
+        const SizedBox(height: 16),
         Text(
           messages[tab] ?? '暂无数据',
           textAlign: TextAlign.center,
-          style: TextStyle(color: Colors.orange.shade600),
+          style: TextStyle(color: colors.onSurface, fontSize: 16),
         ),
       ],
     ),