You will need to use a faux-column technique, using a background image (I have attached one in case you need it). Place that (or your own image) in folder sitesculptor_assets/content/images/ and then reference it in the rule for contentWrapper. In your screen.css at about line 157.
Change:
#outerWrapper #contentWrapper {
overflow: hidden;
}
to:
#outerWrapper #contentWrapper {
background-image: url("../images/content_wrapper_bg.jpg");
background-repeat: repeat-y;
background-position: center top;
overflow: hidden;
}