1. Kaleidoscope (the most common OS 9 theme system extension) runs fine under emulation, so if you just want to _enjoy_ these themes, your best bet is the SheepShaver emulator (in my opinion).
2. Nothing's stopping us from creating an alternative desktop environment for the Mac, such as XQuartz. And then you can build theme support on top of something like that. But most applications wouldn't use that desktop environment for their own UI.
Yea but why use a mac at that point? I don't see anyone on the linux side of things making anything that acknowledges why people use macs in the first place; the entire ecosystem is built to reproduce the IBM PC (...in a unix/like environment). Particularly with its disastrous keybindings and perplexing UI decisions.
Queue a Mac-to-Linux user to contradict this, and there's plenty of them, but we Mac users are truly the dictionary definition of the sunk cost fallacy in more ways than one.
This has been my own observation as well. Although I do have to say, System76's COSMIC DE has been the first one that is actually making me consider going to Linux full time since I've switched to Mac from it. macOS has only been continuing to annoy me more and more lately.
The problem for me is hardware. Asahi isn't there yet, and I'm not giving up my M4 Pro MBP. There's no other laptop on the market that checks all the same boxes. But for now, I've just bypassed the mac's weak points by using Aerospace, and if I'm honest with myself I don't want to give up the ecosystem integration either.
I ask myself that with increasing frequency. Looking at all the Linux options feels like wandering through Akihabara, but one day the macOS that remains will be so unlike the macOS I enjoyed, that I'll jump the fence.
Anyway, if someone does make a new Mac desktop environment, it'd probably be for fun more than anything. Or a Terry Davis scenario.