Firefox already used uxtheme for that. A lot of programs and toolkits do, since the heavyweight HWND-based comctl32 controls aren't suitable for things like browsers because they're slow, hard to render without flicker, there's a limit on how many you can have, they don't play well with effects like opacity and transform3d() and so on. Even Internet Explorer did[1].

I guess uxtheme is unsuitable for win32k lockdown for some reason, probably because it's GDI-based.

[1]: https://devblogs.microsoft.com/oldnewthing/20050211-00/?p=36...