Since you guys are overhauling this system, can I make one small request? Allow any user to use the base SO theme on any site. I personally think that it is a lot cleaner than the other themes, and I quite like how it looks.
I made userscript that replaced all the css files with SO's files and it turned out to be fairly easy at the moment with the current HTML architecture since you guys helpfully used the same naming convention for all your css files. With the new system, it will likely be easier. However, if this was baked into the new themes, that would be even better.
Specifically what I'm thinking is that there would be an option in site settings that is just a checkbox like [_] Use Stack Overflow Theme. If checked, then on page load, the server pulls in the SO.css files instead of site specific ones, and Bob's your uncle.
Would this be feasible to implement?