Dark Oranage theme for Emerald.
If you do not have Emerald installed already, then open up a terminal and paste in this command:
sudo apt-get install emerald
Then go to System > Preferences > Emerald Theme Manager. Also if the theme is not applied automatically, go to System > Preferences > Startup Applications, click add and type 'Emerald theme' for the name and 'emerald --replace' as the command.