How to manually put a background image on a tumblr layout that happens to have that feature disabled?

So I’ve been trying to figure this out for 3 hours and I am super frustrated… I’ve been trying to manually edit the html code of the Lightweight layout by Artur Kim on tumblr to include my own background image, but nothing is working. I assume this is because this particular layout has that ability disabled (I was able to add my own background to my last layout with no problems), but there /has/ to be a way to edit it in html, right? Is there some sort of code in the html that is preventing me from changing the background image that I can get rid of? Thanks in advance :]