I couldn't disagree more. If the user wants the intended experience, they will not apply a different theme for your app. The whole reason a user has applied a different theme on your app is because they disagree with your choices and that's totally fine. What's next, asking Mozilla to take down user style sheets and extensions that restyle web apps? This article is antithetical to the whole cultural foundation that G…
They're specifically talking to distros in the letter, not end users. If you want to install your own custom theme, fine. But if you're a distro shipping a custom theme, when it breaks a 3rd party app, the app gets the blame, not the distro.
This is what I take issue with as a user. A major reason why I prefer GTK is exactly because I can get a homogenous look without having to tweak every single app individually. There are definitely issues with the way this is handled (and I'm TBH still confused about squaring Qt and GTK styles), but let's not throw out the baby with the bathwater.