title: Change gray overlay background of BottomSheet
body: <p>Is there a way to change the overlay background color when using <code>showModalBottomSheet</code>?</p>
  
  <p>Right now the color is always a gray color, but I want to use a different color like green as shown below.</p>
  
  <p><a href="https://i.stack.imgur.com/Q6MBE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Q6MBE.png" alt="changing color"></a></p>
  
  <p>Here is the code used in the demo for reference</p>
  
  <pre><code>        showModalBottomSheet&lt;void&gt;(context: context, builder: (BuildContext context) {
            return Container(
              child: Padding(
                padding: const EdgeInsets.all(32.0),
                child: Text('This is the modal bottom sheet. Tap anywhere to dismiss.',
                  textAlign: TextAlign.center,
                  style: TextStyle(
                    color: Theme.of(context).accentColor,
                    fontSize: 24.0
                  )
                )
              )
            );
          });
  </code></pre>