So I decided I'd give Ubuntu MATE a shot. I've tried to install many themes and they simply don't work. Even the ones that literally say that they support MATE. I've installed them the same way as on any other desktop environment by extracting them and moving them into the .themes folder in the home directory. I have no idea what the issue is. I installed the gtk2-engines-murrine gtk2-engines-pixbuf packages. It seems like every theme I've tried doesn't work.
Try to empty .themes and .icons and then go to Software Boutique -> Fixes and fix the broken packages. You also could try to restore the original windows in Mate Tweak.
When I tried 18.10, the themes worked fine. I could imagine that one of the themes that you downloaded broke the system. If that has happened, then you might consider a reinstallation of Ubuntu Mate.
I tried the Mojave theme too and I downloaded all packages from that link.
