Well, if we look at the official site and StackOverflow side-by-side it almost seems as if they were built by the same designer. StackExchange sites seem to be fairly minimalist and clean, which reflects a certain image - mainly that the content is more important than the choice of background image. There is a professional and educated elegance to white/light backgrounds and mostly monochrome interfaces and this also allows splashes of color to truly stand out so long as they are sparingly used.
In fact, it seems that most of the various StackExchange sites are essentially StackOverflow clones with slight changes to the color scheme, which helps tie the disparate sites together as a community and I for one think we would do well to continue this trend.
For color scheme I recommend we continue the use of pastel blues typically seen on the various official Bitcoin sites and our "color shocks" should be the bright orange of the official logo. Links and other elements deemed important but not worthy of "bright orange status" can use a darkened gold color which ties in quite nicely to certain features of the client and other common Bitcoin icons and logos - the color of the links on the "sketchy" theme we're currently using would work quite nicely actually.
Another important consideration is the logo. The default Bitcoin logo might work well enough, though it doesn't seem to "fit in" with many of the other StackExchange site logos. Glancing down the list, though, I do see a few logos that we could say the same of so perhaps that's not as big an issue as I first thought.