Remember that app in the 1990s that allowed heavy-duty customization of the OS? Was it called Kaleidoscope? Can't remember.

I know that it could do things like apply a "steampunk" theme, where all the scrollbars turned into brass poles, with green leather handles for the thumb button.

Some of the themes were vomit-inducing (most of them, actually), but it did have a couple of great ones.