for some reason Cinnamon defaults to the green-teal-ish Mint-Y-Dark theme, regardless of which one you pick under "controls".
This used to work just fine, and i couldnt find anything in Kvantum or QT5 Settings to resolve it.
/edit: Tried reinstalling the whole theme package of course, to no avail.
/edit2: Looks like a problem with the package itself, that got updated yesterday.
The "color" values in the .css files on my backups are completely different than the current ones
that came with the updated package.
Just paving the backup over the current theme solved it for now, but the package needs to be fixed.