Worst case it is the same as KDE, you have to set some environment variables and it still may not work correctly with some apps because third-party theming is inherently unreliable. The tweak tool method was also an unsupported third-party thing with most of the same limitations. It has been in a broken state for a while but you probably just didn't notice it because you weren't unlucky enough to end up in a situation where it affected you.
I'd still like to emphasize my other comments though, third-party theming is quite broken on every single platform I've ever used and I suggest against using it if you want to avoid apps randomly breaking. If you plan to continue that route then you may have to get more involved in development to the point where you can fix theme bugs and patch the apps without asking other people for help. That's been my experience with third-party themes anyway, they are essentially hacking the apps and so are not really for average users to mess with.