What's happening this time with GNOME?
They have done pretty much the opposite - they added a proper API of theming instead of having a stylesheet override. GTK didn't really have themes before, so this is quite nice.
That's not theming, that's just changing accent colors in the same theme. If I don't like dark background color, I can't change it. If I don't like the light background color, I can't change that either. All I can do is change accent colors.
The entire GTK and GNOME ecosystem is heading towards Android like customization and theming, which is basically non-existent at this point.
What?
GTK1 and GTK2 had zillions of actual themes and engines.
Now, there's just a re-coloring API where I can't change the dark background if I don't like it and that's "real" theming.