Just watched the Linux Gamer's video on "Stop Theming My App". He makes a comparison how, if distros started repackaging ffmpeg to work in different ways, everyone would freak out, and that it's the same with GTK3 apps. The big issue with this is that it would only make sense, if every ffmpeg competitor had established that distro specific changes were normal. In every GTK3 competitor I know of, theming is the established standard.