What are you trying to change exactly -- the body selector?
Currently it is set to #FFFFFF (white) both in your embedded CSS and in one of the other style sheets. You have links to quite a few CSS files and some of them are 404 missing.
Trying to add a background.png image so the sides of the page would have color and the middle would remain white.
Open style-default-menus-light.css. At around line 505, you have this style which effects the background.
padding: 0 0 15px;
background: #ffffff; /**change this**/