It's in the individual pages, not the common.css

This Forsaken's page code is usually found at the top of the article:

<div align="center" style="position:fixed; z-index:-1; top:50; left:0; right:0; bottom:0;">[[File:Banner 3.jpg|2100px]]</div>

It appears that there is no such code on the Funkipedia Mods page.

PS: If you cannot find the code in the common.css, sometimes the source (aka the page itself) can guide you where the code is happening. However, I wouldn't blame you if you checked the page thoroughly and couldn't find it.