Gnome theme (gtk 3.18), based upon Adwaita dark skin
Compatible with Gtk 3.18.
For newer versions of gtk (3.20+), use the successor Obsidian 2:
https://github.com/madmaxms/theme-obsidian-2
Clone or download the repo and move the containing folder Obsidian-1 to your ~/.themes directory, or to /usr/share/themes (needs root access).