Even if the programs you used all supported it, there'd still be no getting around the fact that you'd need to learn some kind of framework or system to modify

Even if the programs you used all supported it, there'd still be no getting around the fact that you'd need to learn some kind of framework or system to modify the UI to your liking. I guess we'd need someone to create one that was intuitive to use and very easy for programs to support, then it'd have to be popular enough with programmers that they'd actually use it. The closest thing I've seen would be websites, since we can remove elements or use customer CSS to change them. Maybe GTK, and those interfaces aren't exactly pretty. It'd probably have to be free, fast, secure, simple, attractive, flexible, powerful, able to work with all kinds of platforms/screens/inputs, and make creating GUIs easier for programmers to create in general (seems like there's a need there), but even then it'd have to contend with companies who want control over what users see, artists who think they know better than everyone else, and support teams that want documentation full of meaningful screenshots.