Allow me to be the first to say this is a pretty lame cop-out that has an obvious, albeit difficult programmatic solution.
Have you guys considered that cool themes might be one of the things that actually set you apart from other generic Q&A sites and having awesome and complete themes like what Jin dreamed up on Christianity.SE make for a better user experience than being able to see stuff when I shrink the web page?
Also, does a lack of an expectation of a cool theme (because something that looks like what I can do in 4 minutes on a wordpress blog is not cool) affect the morale of the core group of users on the newer sites? I don't think I would have tried half as hard on Christianity.SE if I knew that graduation meant "beta" is going away.
All I'm saying is you're sucking the leaven out of the cake and you probably will only wind up with flat cake, viewable on a tiny screen.
On a completely opposite note, is there a responsive design for big screens? Like 4K projections?