This is only possible with theme patch, however. And it probably will look better than Windows 11. At first I thought it's not possible, but it is. Well, now I can give myself and others a definitive answer of how to do it.

I went on a few tangents, however, like learning a bit of assembly and injection into other processes, a bit of JavaScript/CSS to style Firefox too, made custom low level API registry access library, etc.

I was focused on this all this time, almost all day long. I can't believe that 2 months actually passed since I embarked on the quest to make Windows 10 100% Dark Mode, in every window in the system.