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.
What we are most likely seeing is push-back over some of the excesses of themes. Branding is likely part of that, but the astounding number of controls likely played a negative role in the user experience as well.