I've talked to Microsoft a lot on this issue. The main issue is tech support. They don't want to have to support themes.
But if they let a third party do it and there's problems, they'll come to us, not Microsoft.
I've been playing with some of the .msstyles files floating around and even in this early stage problems are showing up. I had one where the items on a dialog weren't painted until I moved another window over it. Another with screen dirt in the toolbars. Nothing functional, just cosmetic problems but it's already begun.
I think I said elsewhere that time is on WindowBlinds's side. As time goes on, it continues to evolve and progress. But msthemes won't.
That said, Stardock has to PROVE it's worth the trouble to install WindowBlinds. it has to PROVE that it's as fast or faster. It has to be better. The free version has to be better and the $20 enhanced version has to be so much better that it's worth it.
That's the challenge we face.