That site isn’t RedHat/GNOME. From the bottom of the letter:
Note: Even though some of us are Foundation members or work on GNOME, these are our personal views as individuals, and not those of the GNOME Project, the GNOME Foundation, or our employers.
They aren’t against user theming. Again, from the site:
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.
They’re against distributions shipping custom stylesheets by default. Which makes sense! If a user has a stock installation, and an app looks broken, they aren’t going to assume the distribution messed it up. They might not even know that the distribution changed the theme. It can also cause confusion for users when their app doesn’t look like the screenshots from the developer. These cause issues for app developers.
That’s it. That’s all the letter is saying. It’s not a crusade against you theming, it’s asking for theming not to be done by distributions.
(P.S. I don’t intend for this to be aggressive. Just wanted to explain a bit more, because the name does sound… not great.)