The theming they dislike isn't the theming their users are doing. Their biggest issue is distros packaging their own theme, which can easily break an entire suit of applications without the user knowing it's because their distro decided to override the colours or mess with widths. As the page you linked says:

> If you like to tinker with your own system, that’s fine with us. However, if you change things like stylesheets and icons, you should be aware that you’re in unsupported territory. Any issues you encounter should be reported to the theme developer, not the app developer.

Theming Gnome by editing the default style sheet is akin to distro theming the web by modifying Firefoz's default style sheet. Tons of websites will break in weird and interesting ways but the user won't know to blame the distro, because they never touched user.css or even know it's possible to change the default.

The same breakage also happens on KDE and Qt applications (and more, I have seen Eclipse launch with black text on a black background when dark mode is enabled) but the developers don't seem to care as much, or are less bothered by the false bug reports. I've also experienced the "this doesn't look anything like the screenshots" feeling because of the theme I have applied, but at least I know it's my own fault.

To be fair, this one is specially to address the issue of a distro theming an app, breaking it, and that leading to a bug report at the upstream even though it's the distro at fault.

If a user is willing to take responsibility for breakage and such, then I'm not sure the devs are against theming as such.

> If you like to tinker with your own system, that’s fine with us. However, if you change things like stylesheets and icons, you should be aware that you’re in unsupported territory. Any issues you encounter should be reported to the theme developer, not the app developer.