It blows my mind how diverse these are, and how diverse their creators were. One single artist, Martha Royer, made over two hundred themes: https://macthemes.garden/authors/martha-royer/ (They're not all amazing in quality, but the sheer industriousness is staggering.)

I was lurking around the equivalent Windows community in roughly the same era (well, a couple of years later) and it was nothing like this. Far fewer people had the patience for WindowBlinds (the Kaleidoscope equivalent) or deep OS modding, and they tended to all be the same few types of person with more-or-less the same tastes—mostly the kind of guy who thought that an RGB fan in a brushed aluminum tower PC was the height of self-expression. (Basic Windows Plus themes were way more primitive than what was possible with the right tools.) It's astonishing to see what looks like the entirety of the pre-dot-com-crash web's wonderful, weird diversity reproduced in perfect miniature over in the Mac space.

Although I keep scrolling, and I haven't found a legit NeXT theme yet. There are a few that get close but nothing with the actual UI assets. Maybe this is an opportunity...

EDIT: For those curious, here's roughly what themes on OS X looked like in the 2000s: https://macgui.com/downloads/?cat_id=10