Since I see this argument starting to resurface on my timeline: There isn't a correct answer for whether theming is broken or not.
Some apps intentionally or not will work fine with theming, others will benefit from being designed against a specific theme. For those it may be beneficial for some desktops to discourage switching themes.
The question should be asked on a case by case basis, not globally.
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).