Sorry to be so harsh but, the introduction of themes signals to me that the developers have thrown their hands up in defeat. The alternative is to improve the look and feel of the core design.
You guys have managed to rip off / borrow stackoverflow's look and feel ( which is good ) and moved the all text a few pixels off center, shade icons that were unshaded, resize GUI elements. If you going to take so many GUI elements, why not take everything?
But you offered themes as a solution to a statement about UI quality, which is it not. Also you don't have to do what your users ask for, saying no to bad ideas is important.
Your not alone, the chromium team should have said no to themes as well.
Well it's open source dude, if you think you can do better at UI quality feel free to give it a try. And thanks for the advice... Also, theming is a good features, not all out users want to have the same UI. They can also fully customize the CSS. And for the matter, SO UI is far from perfect.
I was too harsh earlier. The main point I'd like to make is that themes are independent of the core UI refinements that need to happen. Shapado faces 6+ months of work before it can have quality of SO.
I support open source software, but who is going to sit down and do this painstaking work. It's easier to work on theming or mongodb, rather than make the titles spaced correctly ( for example ).