GTK has more and more become a stripped-down toolkit that requires you to write or use a "platform library" (like libadwaita) in order to do useful things. Judging by the deprecations in GTK4, as well as statements from GTK developers, GTK5 will be even more stripped-down. This just makes it harder for non-GNOME projects to use it.

On top of that, each major GTK release comes with drastic changes to how classes of widgets work, which for smaller teams could mean years of work to migrate to the new version. I don't begrudge the GTK developers their ability to make all of these sorts of changes; after all, I'm not paying them for any kind of support or feature set. But it's still frustrating for smaller teams that don't realistically have the ability to take on the maintenance burden of an entire UI toolkit.

Gnome does not expose this at all (except for the built in dark mode) and they actively discourage any kind of theming. Gnome 47 will finally support accent colors, which they reluctantly implemented after it became a freedesktop standard and most distros were patching it in anyway.