You forgot the ; after the height and width properties in your CSS and you would probably want to use "center top", not "center center", but besides that, I think what you need to do is edit the templates and add that background as an image, like in the tutorial I linked you to. See their page here -
http://www.onsitus.it/css-tutorials/sfondo-100-percent/ Isn't that what you want? Look at the page source - the background is an image tag added to the page.