In the last century, there was an app for Apple's Mac OS (before MacOSX), called Kaleidoscope[0]. It allowed massive customization.

Fun, but most themes were basically unusable.

[0] https://en.wikipedia.org/wiki/Kaleidoscope_(software)

Some samples: https://web.archive.org/web/20110709175639fw_/http://www.kal...

(The one I used the most, was Ozymandias II).