我开始使用材质3来进行Flutter。但是，即使使用默认的基本计数器应用程序，当我添加：

useMaterial3: true,

应用程序栏变白色，没有背景。即使我设置：配色方案种子
下面是我的代码：

// This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorSchemeSeed: Colors.red,
        useMaterial3: true,

      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }

为什么应用程序栏变成了白色背景？

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
   
    return Scaffold(
      appBar: AppBar(
    
        title: Text(widget.title),
      ),
      body: Center(
   
        child: Column(
         
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), 
    );
  }
}