title: How to change the background color of PopupMenuItem under the showMenu?
body: <p>I don't use PopupMenuButton.</p>
  
  <p>Because in my case, showMenu would be more appropriate.</p>
  
  <pre><code>onLongPress: () =&gt; _onRoomLongPressed(context, _tappedPosition, model, room),
  </code></pre>
  
  <pre><code>void _onRoomLongPressed(BuildContext context, Offset tappedPosition,
      RoomsModel model, Room room) {
    final RenderBox overlay = Overlay.of(context).context.findRenderObject();
  
    showMenu(
      context: context,
      position: RelativeRect.fromRect(
          tappedPosition &amp; Size.zero, Offset.zero &amp; overlay.size),
      items: [
        PopupMenuItem(
          value: 'delete',
          child: Text('delete'),
        ),
      ],
    ).then((String value) {
      switch (value) {
        case 'delete':
          model.remove(room);
          break;
      }
    });
  }
  </code></pre>
  
  <p>Don't change the theme of <code>MaterialApp()</code>.</p>
  
  <p>I expect that the background color of <code>PopupMenuItem</code> to be white and the <code>Text</code> color black.</p>