I try to theme it a bit but I don't add javascript as it slows some members downloading and I've seen many complaining about it. I also try to do something season related, as long as I find people up for it.
I'd say keep it generalized. More seasonal unless it's a national holiday (ie thanksgiving, 4th of july in america of course). Many winter holidays are religion based and you could offend if you have a diverse userbase so I would stick to just a winter theme (snowflakes and snowmen) rather than Christmas trees and Manorah's.
On my forum, we aren't doing anything special per-say, we're just having a random get-together of our RP characters and hanging out. (in a spam rp board, 'course)
On other forums, to my knowledge, nothing special happens.