Some people say that computers have become so standardized that they are just a commodity. But consider how much PC interfaces have changed. This goes far beyond changing colors and fonts.
Themes can drastically change the appearance of an application, and even change the functionality.
Proponents tout them as a way to help people bond with their software. Themes began with media players such as Winamp and RealJukebox, and now seem to be spreading, kudzu-like, into other application categories and operating systems themselves. The ability to theme is an in-demand feature that can differentiate a piece of software. That role is enhanced by uniformity. Consider driving a car.