It EXPLICITLY says that it does not talk about users changing things on their machines, but wants it to be something the user sets instead of a distro applying by default.
"However, if you change things like stylesheets and icons, you should be aware that you’re in unsupported territory."
Unsupported territory, for changing a desktop icon, or adding a higher or lower contrast stylesheet? Wow. That's pompous and absurd for me. Doesn't sound like it's explicitly made a reasonable exception though, just piles on a layer of entitlement for a damn brand.
Read the banner at the top of the page again. It doesn’t ask users to stop customizing, and it doesn’t say there shouldn’t be distros with custom theming. It just asks distros to stop shipping broken apps as a result of theming, and it asserts that if that’s the route you want to go, then you are on your own.
Reading carefully between the lines, I think they actually do hint at that:
> Please don’t theme our apps
> This is why we ask respectfully that our applications not be themed.
> On a platform level, we believe GTK should stop forcing a single stylesheet on all apps by default.
> If you are a distribution who changes the system stylesheet and icons, please reconsider this decision.
> If you do, we ask that you please stop theming our apps.