主题：

1. 设置

theme: ThemeData(
        backgroundColor: Colors.white,
        primarySwatch: Colors.purple,
        splashColor: Colors.transparent,
        highlightColor: Colors.transparent,
      ),

2. 继承

Scaffold(
      body: Theme(
        data:Theme.of(context).copyWith(
          primaryColor: Colors.black
        ),
        child:ThemeDemo()
      ),
    );