The furthest back background on my elemental theme shows this dotted grid pattern. I can change the custom css in the design theme customizations but all that does is change it to a black background with a white dot grid pattern. How in the world do I make the background a solid color?

I'm talking about the furthest background. The ones in the front for the header, body, and footer are normal, but if I have an ad pop up between one of these sections it moves them out of the way to reveal the true background which is some hideous dotted pattern. Please, I know someone has to know how to fix this easily, someone help.

I attached a screenshot of what the background looks like, it's just a small strip of the background pattern.