Edge is a nice and fresh GTK theme by a Hungarian author, Márton Kardos. It has a light and a dark variant as well. At the moment it only supports GTK 3.16 but the new version will support GTK 3.20 and it will also feature a redesigned Nautilus file browser.
Screenshots
How to install?
- Download your preferred version of the theme
- Unzip it and move it inside your ~/.themes folder
- Set it as your theme by using for example Gnome Tweak Tool