change the background color of all pages flutter code example

Example 1: how to change appbar color in flutter

appBar: AppBar(
      title: const Text('Example'),
      backgroundColor: Colors.black,
    ),

Example 2: how to set opacity of background color in flutter

Opacity(
          opacity: 0.5,
          child: Container(
          color: Colors.red,
          width: 200,
          height: 200,
  ),
        ),