Orta is a sleek looking GTK theme. It can be installed in Ubuntu from the PPA:
sudo add-apt-repository ppa:nikount/orta-desktop
sudo apt-get update
sudo apt-get install orta-theme
The Orta Settings Manager will also be installed for you to further customize the theme.
Furthermore, with the PPA, you can also install the following packages:
sudo apt-get install orta-emerald-decorators
sudo apt-get install orta-xfwm4-decorators
If you are not running Ubuntu, you can get the Orta GTK theme here.
Images by SkiesOfAzel.