to change the page background (the part OUTSIDE the main body of the store) look at the body section in the style sheet... change the following value to what color you want or add an image as shown in the next example..

Code:
body {background:#999;}


to change the store background (the part inside the store boundaries) change the following:
(you can use the color value, or add the background image, or both... )

Code:
#mainWrapper {background: #fdfde9 url("../images/your_background.gif"); }