Gnome theme (gtk 3.22), based upon Adwaita-Maia dark skin
Compatible with Gtk 3.22.
For Cinnamon 3.2 and older versions of gtk (3.18 and below), use the predecessor Obsidian 1:
https://github.com/madmaxms/theme-obsidian-1
Clone or download the repo and move the containing folder Obsidian-2 to your ~/.themes directory, or to /usr/share/themes (needs root access).