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.
GNOME is not a web page, so you cannot “customise” the appearances of the shell and the applications like you would in the CSS Zen Garden.
Both GNOME Shell and GTK use CSS for styling purposes; the supported CSS is not the same as the web CSS, though GTK strives to conformance.
You can check the supported subset (and custom extensions) in the GTK documentation:
You can also use the GTK Inspector to inspect a GTK application and even modify the CSS at run time.
Still, you have to be mindful that applications are shipped with their own styles; changing the style of an application behind its back is an excellent recipe for breaking the overall UI.