You can change the css of vb through styles > main css

You can select to store the css per style, as a file, or in the page.

What are the benefits, downsides, etc?

My understanding is that its better to store it in the file system since the database wont need to query the information repeatedly, but I think you need to chmod the directory which not all people know how to do.

Bigger sites will see more of a pay off from moving it to the file system over keeping it in the DB.

I store them as files, because else your pages increase in size, and now they can cache the css file.

Result: Faster forum, less bandwith.

And less crap for the spiders to have to wade through beofre they can see your content.

i store mine as a file for the reasons previously mentioned

Same as above...file.

Database - does anyone know if vB will work "right" if it's running across multiple servers and the CSS is in files? It's the only reason I've not switched it over so far, but attachments / avatars don't work that way...

I store them as files, because else your pages increase in size, and now they can cache the css file.

Result: Faster forum, less bandwith.
Agreed. I store as pages as well, obviously :P .