"Kaleidoscope" [1] needs to be mentioned here as well. A brilliant shareware to apply a "scheme" (graphical theme) to classic MacOS. Wanted your UI to look like Windows, Copland [2] or BeOS? Kaleidoscope allowed you to do that.

Here's a link to previews of some popular schemes (from the Internet Archive): https://web.archive.org/web/20060925184709fw_/http://www.kal...

Sadly, modern OS will probably never allow this level of customizability...

[1] https://en.wikipedia.org/wiki/Kaleidoscope_(software) [2] https://en.wikipedia.org/wiki/Copland_(operating_system)