I’m working on the 30 day trial and don’t have a lot of time to customize themes.  Is there a generic theme that works fairly well across the site and modules (forums, wiki, etc)?

EE itself has some default themes-- in my opinion, they look pretty good.  You may also want to check out the Template Library--they have some nice ones there.

Hi, xiola—welcome to the forums!  Vanceone has already pointed you to our template library.  From memory, I believe The Linguist released a forum theme for Logical Blocks; but I am not sure about a wiki theme.  Most people that do customize tend to use their own designs throughout the site, and I can’t recall seeing a full collection released.

That said, you can certainly use the demo to look and do some quick customizations on the forums, at least.  That will give you a feel for how they work and then you can take your time customizing them if you decide to go ahead with ExpressionEngine.