There is an open letter from Gnome-developers that distributions shouldn’t theme apps (icons, window themes etc). They acknowledge though that if you want to do it on your system but that is ok and that you are in unsupported areas then. The reasoning is that all theming in Gnome is only a hack since there is no theming -API and things can and will break.
I am a Linux-user and I am a Gnome-user. I like to theme my desktop and think that it is one of the great things on the Linux-desktop: I can make it look like I want it. Some desktops work better for that, some are not so good. Gnome is inherently bad for this. Regularly themes break with Gnome-upgrades. And I just couldn’t understand why the Gnome-devs just not stabilize how elements are called and hand out documentation. After all, gnome themes are stylesheets and graphics.
And theming GDM is really hard. I never really figured out how I do it non-destructibly.
Apparently it is not the intention of Gnome to be themable. I have to say that as a user I’d rather like to see an open letter from developers of GTK-apps that Gnome gets a theming-API than asking distribution develops to stop theming their apps. One way or another, you have to do the design work and if there would be a stable API, theme developers could use that. Then the app-dev would style her app, the theme-dev develops her theme and when there is a bug with a theme, you file it with the theme-dev, not the app-dev.
And Adwaita got a lot better with Gnome 3.32 but I prefer themes like Arc, Yaru, North and others more.