Earlier quoted context omitted.
Why should you be able to change themes or fonts? OS X (iOS as well and I think Windows Phone 7, too) doesn't allow you to do that. Those two don't seem like required features to me.
Many people do desire the feature to change themes. A big reason to choose open source in the first place is the flexibility in customization that it brings. It's the opposite of the "Apple mindset" that there's one good way and that's the Steve way.
I think one of the things that Gnome has achieved over the years is removing lots of configuration options, and battling the hostile responses of a vocal minority who would choose complexity for everyone so that they can tweak any minor detail. Remember the backlash when Gnome replaced Sawfish with Metacity as its default window manager, and users could no longer script the window manager in a dialect of Lisp?
I'm all for options to personalise the user interface where it make sense, but if I have to start configuring your software in order to make it usable for the job it's supposed to be designed for, you've failed.