Do you want to create a single forum like the one here at CSS-Tricks or you mean actually create a full forum-hosting system? The former is easy but the latter would be fairly complex. Any old CMS (content management system) would give you the ability to make a single forum and there are stand alone forum packages as well. No idea how you would go about making and hosting a mix your own flavor forum site, but I’ll put on my spelunking hat and speculate about what it would take. Yeah, I know spelunking is exploring caves, but whatever, sounded funny when I thought of it :) .
This is all guesswork and someone else probably knows far more than I do, but here goes. You would need to be able to give the users permission to create their own forums (not a big deal), but the tricky part is allowing theming controls to be accessible to the users (a much bigger deal.) You probably would also need at least a Virtual Private Server to host the site, which would be a step up cost-wise but give you memory and bandwidth you’d need. Dedicated forum software might be the way to go, or you could try with an open source CMS, but I think you are in for some tweaking of any of these options if you want forum members to be able to theme their own forums.
