I'm trying to change this page's background, like a page i saw from a different Wiki.

Attemp #1: I follow this page, actually i just copy 100% the code and nothing happen. 
Attemp #2: Wait, i read the thing wrong, don't mind me.
Attemp #13: Finally, now i know how to add a background that fit the whole screen, sounds like a 3 years old child.