I'd probably pay to skin Windows if it worked really well (fast and no flashes of unskinned stuff). I've wanted to tinker with that for ages but I don't even know where to begin.
Windhawk[1] has some plugins for styling (using XAML). I don't really style anything apart from removing the "Recommended" section from the start menu, so I'm not sure how fast styles get applied.
I was remembering this from when? 20 years ago? I was using Linux already but have to use Windows at work and couldn't tolerate not being able to change things to my liking
[1]: https://windhawk.net