I have been a huge fan of GIMP for years and I created numerous GTK and Metacity themes when I was using Linux. Now that I have a Mac mini (Late 2014) running macOS Mojave, I would like to theme it to look more like OS X Mountain Lion. I still think Mountain Lion was the best looking OS X release. Creating the graphics won't be a problem, I'll just spend some time in GIMP and whip up something to my liking. The issue I face is being new to macOS and not knowing which files I would need to replace. Can anyone give me any pointers?