DeLorean-Dark is an amazing GTK3 dark theme for Ubuntu/Linux Mint systems. The theme has been updated for Gnome Shell 3.6, that means you can now install it in Ubuntu 12.10 or Linux Mint 14 (Nadia).
Linux Mint 14
To install DeLorean-Dark in Ubuntu 12.10/Linux Mint 14, open the terminal and run these commands:
sudo add-apt-repository ppa:killhellokitty/themes.ppa
sudo apt-get update
sudo apt-get install delorean-dark-theme-3.6
To enable it, use any of your favorite programs (MyUnity, Gnome Tweak Tool, etc.). The icons used in the Ubuntu screenshot are from NITRUX OS Icons. Credits for this theme go here.