I don't see any styles which could override the background color that you specified for the "html, body" selector. It may be that either X-Cart, or your browser cached the color. Try to clean both the caches.
Also, try to use the "Inspect this element" feature of your browser to understand what CSS rules style elements on your website.
One more useful tool is "Web Developer Tools" extension for Firefox and Google Chrome browsers:
http://chrispederick.com/work/web-developer/