Mario Guzman 🏳️‍🌈, I recently found out that it's been possible for a while to load an NSAppearance from a .car file. Problem is, there aren't any of those appearance files besides the ones shipped with macOS itself. I pulled one out of my older Mac running Mojave, and it actually worked. These are full-blown skins and I feel like there's a huge potential.

I've also extracted all the assets from 10.9. The "only" thing left to do is to put them together into a correctly formatted .car file.