Orta is great looking theme with light weight for your desktop.
Install Orta GTK Theme using Ubuntu PPA
To add the ppa and install the theme and the settings manager, type this on a terminal :

sudo add-apt-repository ppa:nikount/orta-desktop
sudo apt-get update
sudo apt-get install orta-theme

To install the emerald decorators :

sudo apt-get install orta-emerald-decorators

To install the Xfwm4 decorators :

sudo apt-get install orta-xfwm4-decorators

You can find the Orta Settings Manager under the Preferences category of your Main Menu.
Screenshot:

Enjoi! 🙂
For more info Click HERE