Boxes behind posts etc.

I’m trying to customize the theme “Decayed” (http://themes.wordpress.net/columns/2-columns/3167/decayed-10/) by changing the background and making it fixed. However, because I want part of the fixed background to be white and I’d sill like my text to be readable, I want to have translucent boxes, for lack of a better term, behind the sidebar, posts etc. so that the text will be readable but the background will still be visible underneath the text. I don’t really know how to put it any better… It’s the kind of thing seen often on MySpace. Anyway, I’ve figured out how to make my background fixed but adding any kind of background to the sidebar and posts and then changing the opacity is pretty much beyond my knowledge. I also don’t quite know how to phrase it to search properly for what I want. Help?

Thanks in advance!