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.
(I used to work on the blog author’s team, and in fact dealt with some of the COM issues mentioned in the post)
You’re right, uxtheme can’t be called directly from content because of win32k dependencies. One option could be for content to request the parent to draw those controls, but as you can imagine there are drawbacks to doing that. As usual, there are a lot of trade offs involved.
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...