Thanks Dmitry!
However, the code results in an "illegal qualifier" for:
NSNotificationCenter.defaultCenter.addObserver_selector_name_object
Awesome, your .plist suggestion works! The Dark theme is applied right away at startup, but (obviously when looking at the code) your MacOSFormProps is not catching that and always assumes we're starting with using the Aqua theme. Not intended as criticism, just thought I'd mention it.
I'll patiently wait for your Cocoa WS modifications.
P.s. seems your Mac may still be Mojave suitable with a little patcher tool (
http://dosdude1.com/mojave/) - not sure how reliable this will be with the final release of course.