add linear gradient to scaffold in flutter code example

Example: add gradient bg to container in flutter

body: Container(
  decoration: BoxDecoration(
    gradient: LinearGradient(
      colors: [Colors.green, Colors.blue])
  ),
  child: Center(
    child: Text('Gradients are cool!',
      style: TextStyle(
        fontSize: 35,
        color: Colors.white,
      ),
    ),
  ),
),