title: flutter: how to set background for a sliverlist
body: <p>I've develop an app.There is a page like this:</p>
  
  <p><a href="https://i.stack.imgur.com/okh0q.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/okh0q.jpg" alt="screenshot"></a></p>
  
  <p>If there are more comments, the picture and comment list can be scroll.So I put them in a CustomScrollView.The question is how can I set background for the SliverList?</p>