GTK is nice but too thick. There is a repo containing CSS for GTK3 to be more compact.
I could not find the same for GTK4.
Gradience from Flathub allows to load custom CSS. The GTK3 one did not change anything.
I use Fedora Kinoite (KDE) and all GTK apps are Flatpaks.
It also seems to poke a hole in all Flatpak apps, giving them access to GTK3 themes.
Thanks, maybe it is not even possible? Should ask GNOME devs…