People be claiming that "it's just not possible to have a UI toolkit that's both reliable and themeable outside of core OS styles" and yet none of them ever seem to mention the XP Zune theme which did precisely that

For those not familiar: the Zune theme was a theme released by Microsoft for Windows XP, to promote their (ultimately failed) line of MP3 players.

Crucially, it was released late, relatively obscure, and most developers wouldn't have known about its existence. From a QA perspective, it effectively functioned as a third-party theme despite being published by Microsoft.