Elementary Dark GTK3 theme has been updated for Ubuntu 12.04 and Gnome 3.4.
Made by Satya, the theme is inspired by default Elementary OS theme but it comes as a darker variant.
Download Elementary Dark GTK3 Theme
To install from PPA, run the commands below:
sudo add-apt-repository ppa:satyajit-happy/themes
sudo apt-get update && sudo apt-get install elementary-dark-theme
Use Gnome Tweak Tool or Ubuntu Tweak to switch your GTK3 themes.