How does GNOME themes work
The GNOME themes file is essentially a CSS file, done, you can stop reading. There are a few more details you may want to hear about. The theme files describe what your desktop looks like but that is not all. It also contains the artwork needed for it to work. You make all your configuration in the CSS file of your theme. A caveat is that much of the look comes from the GTK 2.0 and GTK 3.0 themes. Another issue is that the standard setup does not allow your own themes, you need to download the User Theme extension to use your own theme. The reason is that the theme files delivered with your distribution are compiled from gresource files. This integrates the themes closer to the development process but has the drawback that making your own requires more programming skills. Documentation is also scarce, for users that is. Development documentation is plentiful.
How does GNOME themes work
Linux Hint published an introduction on how GNOME themes work.
