Hi Rolf,
you are 100% correct. Creating multiple themes has never been an “official” way of styling applications and more of a workaround that seems to work very well.
The way we currently do it, is to create a single compiled stylesheet, and let the application set a class in the body. We have also rewritten the sass-variables to css-variables in order to make this even more powerful such as realtime styling.
checkout the vid:
https://www.youtube.com/watch?v=8Ju2M4A-uxc