Followed the instructions on https://medium.com/@hinddeep.purohit007/supporting-dark-mode-in-ionic-capacitor-8e57fe9dbd47 , dark mode on android works, and also on iOS.
That is, iOS seems a bit eager: When minimizing the app, it seems DarkMode ON/OFF is issued in rapid succession:

I just run set-dark-theme on ON and then set-normal-theme on OFF, so it's not messing up.
Just noting :)