Thanks @Stranik @tobiaseigen! The PR was only just merged around the time you updated!
https://github.com/discourse/discourse/pull/8073
Thanks for the heads-up! I am going to deprecate the theme component with an update (i.e. wipe it out + some console messages).
I originally thought about keeping it so instances on stable can also try out the feature, but I realized it depends on some other new changes in core, so it essentially only works on tests-passed.