In the Mobile studio, you can change the background of the items, without affecting the background of the same item in the Web studio. You can add either different color, or set an image as background.
How to make changes in color and background of the item?
1. Get inside the mobile studio
2. Choose "Widgets"
3. Hover over the desired item and click on the pencil to edit
4. Click "Style" tab and pick on of the first two icons (image below, in red)
This makes the items it look different from the desktop version of your website.