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That's what I thought when I heard Gnome supports custom CSS: that I could customize its looks with many properties like a webpage. I did a bunch of searches about how to inspect gnome elements and you use Looking Glass, but its not quite like browser dev tools which show the whole DOM like this and with a CSS editor. I stopped since it seemed like a lot of work.
As far as I know, and I know very little, I think the custom CSS stuff is for developing a Gnome app. It's for programmers during building, not for regular users during running. Could be wrong though.