You could simply delete the php that calls said backgrounds.
If you open the classic theme, you’ll see they’re non-existant.
Then simply use the existing div’s, add some classes, and syle vis css.
Interesting to note, however, that there are the embedded classes in wp core: .narrowcolumn and .widecolumn. They’re not in any of the theme markup that I can see. These are missing from the classic style.css.
