xTango - Dark is a dark GTK2 and GTK3 theme based on the GreyBird style and the Tango KDE color theme.
How to install the GTK3 theme?
For detailed installation instructions please see the following tutorial created by Softpedia: http://news.softpedia.com/news/How-to-Install-GNOME-Themes-in-Ubuntu-11-10-231213.shtml
How to install the GTK2 theme?
· Right click on your desktop and select Change Desktop Background
· Click on the first tab: Theme
· Drag and drop the theme's archive in the Appearance Preferences window.
· If everything is OK, you will receive a confirmation message and you can activate the theme just by clicking on it.