I'm all for doing a twist for the holidays. But chances are high you're making it very hard for some people to read your site. Sure there's the option of using custom CSS in the browser, then we're getting into the whole 'lol ur half blind, here's an ad hoc solution', kinda similar to 'you're in a wheel chair so use the back door, but wait 10 minutes so we have time to carry you in' kinda thing, which is what Universal Design is all about. There are other things one can do to create a Halloweeny look, like adding little pics here and there, borders, even poisoning the mouse cursor, or similar. Tweeking the CSS sheet like that is to me kinda 'ooo look I can change a color in CSS, I am totally h4xX0r', and I find the idea of changing the original design that drastic a bit unprofessional.
But that's my personal taste. And I am extremely conservative when it come to web site design. I'm all about function anyway
So, you asked, that's my opinion. It might be colored by the fact that I was just over at the Wiki to read an article and the design was annoying
Other than that, the Wiki is really fantastic and I use it a lot. So, kudos anyway, the fact that you even bothered with something like this proves that you devote a lot of time to the site, which is the #1 recipe for a good site.