Admincp
Styles & Templates
Style Manager
Then on the right side of the style u want to configure, drop down the menu and choose Main CSS
Then you can see the first 2, Body & Page Background.
Make sure u make backup of template before changes
Hope this helps.
Main CSS? In vBulletin4, that is deprecated, it does nothing.
Stromba: You need to find StyleVars, then look for doc_background (or similar) in the left column.