After looking a little closer with
Firebug, it appears that the background image and the color behind it are actually defined on line 3 of css/template.css.
To implement a background color instead of the default image with the "micro-dot" pattern, you'll just have to get rid of the image URL and change the color code to whatever you want... or you could just change images/s5_background.jpg if that's easier for you.