how to give shape to card in flutter

Card(
  color: Colors.grey[900],
  shape: RoundedRectangleBorder(
    side: BorderSide(color: Colors.white70, width: 1),
    borderRadius: BorderRadius.circular(10),
  ),
  margin: EdgeInsets.all(20.0),
  child: Container(
    child: Column(
        children: <Widget>[
        ListTile(
            title: Text(
            'example',
            style: TextStyle(fontSize: 18, color: Colors.white),
            ),
        ),
        ],
    ),
  ),
),