There are no themes, just CSS.

Gnome developer “Tobias Bernard” has wrote a lengthily post on his blog about how people have thought that GTK 3 supports theming. This is not true, as there is only CSS stylesheets. These CSS stylesheets were only ever meant to be used by the platform and app developers. The platform stylesheet is called Adwaita. You can check out the full post over on his blog. It’s totally worth reading & he has some other interesting post work checking out too….