[aecsocket]

Interesting, I didn't know there was a theming presence on OS X! I agree with you in that consistency and themability can exist together (and I suppose your example proves that), and that had GNOME decided to prioritize themability we could have had something like that on the Linux desktop. I suppose this is a question of priority and where to allocate effort, rather than what is technically possible and not. Building a UI framework and HIG is already not an easy task, and making it customizable in the way you describe would be an even bigger burden on developers - many of which are, I assume, doing this work for free. But admittedly I haven't looked much into GNOME's funding or organizational structure, so maybe they are capable of it, but just haven't bothered.