CSS Background Sizing

I’m trying to get my background image (simulating columns, instead of using column background colours), to stretch a certain way. But I’m not sure how to go about it.
Basically, if you look at http://tekmonki.com/tindex.php you’ll see the the diag background on either side of the content. I want that part to fill the rest of the window to the left/right regardless of resolution. From what I can find on the web, to achieve this you make a huge-width image and then specify “min-width” and it will stretch it to fit. What I don’t get though is how it knows what size the inner columns are supposed to be. Is there some ratio or something?