It depends on whether you want to change the color or add an image. As kuroi said, you look in your stylesheet for the body tag. In a stock Zencart installation it looks like this:
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #000000;
background-color: #e5edf5;
}
Change the red part to change the color of the background.



