Elegant Grunge messes up with small window

I noticed that if you shrink your browser window or use a small enough resolution so that the the background image of elegant-grunge does not fit on the screen horizontally, parts of the background on the left disappear and the background no longer lines up with the post text and sidebar. This leaves an ugly gray area on the right side and the sidebar content hangs over the edge of the background into this area.

I’m guessing this is probably an issue with the css, but since I have little experience with that, I was wondering if anyone knows what causes this and how to fix it.