Lots of talking around https://stopthemingmy.app
To be honest my stand on this is very easy: If you want to theme apps, then fork the app. Make sure, the themed app is **your app** and people report bugs for it to your issue tracker. Rebranding is fine.
What is rather annoying: Taking in app, theming and breaking it, then point people to the upstream issue tracker… That's just annoying for everyone upstream.