EDIT: DO NOT USE THIS, please use this instead: https://www.gnome-look.org/p/1173216/
because it's basically this, except it's updated for modern GTK3 and actively maintained.
This theme is from 2012 and until around 2016, the gnome developers thought it would be a fun idea to completely break themes with every minor update of GTK 3.
So this theme is completely broken on any version of GTK made shortly after this was uploaded. I'm keeping it here for archival reasons.