title: Setting a body background image in MuiThemeProvider
body: <p>I have a react app with Material UI. I know how to set the theme's default background color on body. However, I want to set a background image instead. How can I accomplish this?</p>
  
  <pre><code>const theme = createMuiTheme({
    palette: {
      background: {
        default: ???
      }
    }
  });
  </code></pre>