> Why isn't your Emacs session running dark-notify itself, reading its output, and switching accordingly?

Thanks for the hint. Maybe I would try that first, if it popped into my head when I was originally writing it; but at this point, it works, and I want less stuff in init.el.

Also I will likely reuse the dark-notify script to change the theme in Terminal.app as well, and Terminal.app doesn't have an ELisp runtime. (Also very weird that Apple's own software doesn't properly support this.)

> But also, I thought ns-do-applescript was stock Emacs?

I don't know. I've switched to macOS only a couple years ago, and the Emacs build I've initially used didn't have it.