If the impetus for this is purely 'nice to have', then it's probably not worth the work to reward ratio. We'd have to maintain several versions of the top bar, which puts some restrictions on future things (icons, text, links) that might go there 'someday'. We'd either have to work harder to come up with things that fit all available style selections, or deal with several versions of the same thing.
Having this functionality available to you now through the use of custom user style sheets (or downloading custom schemes that others have shared) deflates this further, if it really bothered someone purely on the ground of aesthetics, they're not without the means to do something about it.
In order for it to become something that we'd put squarely on the drawing board, usability would have to be the driving motivation. If that was the case, this ceases to be about cosmetics and more about accessibility. It goes from a preference to more of a mode in those circumstances.
I'm not saying we won't need to make exceptions for certain corner cases (I have no idea what sort of sorcery Jin has in mind for that), but they'll be few and far between. Yet, if we did implement styles as a preference, these cases would be even harder to handle.