I've been looking all over the forums for a way to get different backgrounds for each of my forum categories and I couldn't find anything. The one solution I found only worked for vb 3.8 (I have 4.2)
Can you be more specific? Do you mean you want every forum to have a different background on the forumdisplay page? On the main page? I really can't tell from your description what you want.
All the categories and forums have a unique id associated with them - you can see it in the page source. So, you may just write custom CSS that uses that id instead of a class. I've posted about this before.