I occasionally use some of the other "trilogy" sites like SuperUser, but find the themes very distracting. I actually do like the SO theme the best, and since I use that the most, I would prefer having that look on all the other trinity sites. I would request a feature added to your user profile where you could select your CSS theme you prefer for the site you are on.
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There really shouldn't be any reason why you shouldn't be able choose from any of the various themes. Done right thematic elements would be localized in their own CSS file and it would simply be a matter of switching which CSS file gets loaded. It's even possible, using cookies, to do this without requiring any schema mods.
See the jQuery theme switcher widget for ideas.
I'm sure this can be accomplished via greasemonkey, and I suspect that some browsers may allow you to theme the website with a custom CSS without a greasemonkey script.
However, Jeff and Joel are big on branding and user confusion issues, so I doubt they'll make this a built in feature.
When you prepare to ask a question on SU, you know you're not on SO...