Themes can't change in settings at all when changed on KDE Plasma Desktop until closed and reopened

Ah interesting. I was not aware that was a feature, so whatever I had was the default, I guess.

Yeah, it's available in System Settings > Day-Night Cycle, and System Settings > Colors & Themes then you toggle that "Switch to Dark Mode at Night" switch. image It's available in Plasma 6.6.1, not sure how far back it's available though.