Tumblr help fast!

Ok so I found this theme that seems pretty easy to tweek. I pretty much know were I can fit my graphics in this theme to make my own layout. Its just I cant find were the background is. If someone can tell me how to change the background to this theme it would be great. Here it is. http://customthemes.tumblr.com/post/42985574/perfs-by-jacob-bijani