To be clear, from earlier discussions with them I don't think I'm actually disagreeing with the "Linux desktop theming is broken" crowd.
It's just that the fossbros I ranted takes a certain ownership over GNOME which GNOME does not want. They expect amongst all their nobs to fiddle with control over theming.
For many (but not all) apps this expectation of themability creates a significant maintainance burden. Hence they push back strongly against this expectation.
For people who care about, support, or build Free, Libre, and Open Source Software (FLOSS).