Unpopular opinion: optimising for "branding" is what killed theming. Everyone wants to have their own custom colour theme, brand-associated font or button style, which clashes with uniformity of system controls. If everyone renders buttons in a custom way, they bypass the native, theming-capable, accessible-by-default system controls. As long as product managers are happy, it doesn't matter that users aren't.
Theming was HUGE in Linux back in 2005-2010, consistency was paramount, adherence to toolkit was extremely important just like Apple claimed. For Apple it was about delivering a consistent branded experience, for the GNOME and KDE communities it was about being able to consistently customize the experience. In Windows? Always a free-for-all of such heavy customization on the side of the third-party applications that…
it still is, just look at how neat things in https://reddit.com/r/unixporn/ look