Mashup is an amazing and attractive icon theme for Gnome users. This icon theme has many quality icons designed to be professional that will fit best with dark Ubuntu themes. In this tutorial we will help you install and enable this icon theme with simple terminal commands under Ubuntu 11.10/12.04 (Unity).
Mashup (5.8) Installation
You can install and enable Mashup with these commands from the terminal:
wget -O Mashup-5.8.tar.bz2 http://goo.gl/BXfSs
sudo tar jxvf Mashup-5.8.tar.bz2 -C /usr/share/icons/
gsettings set org.gnome.desktop.interface icon-theme 'Mashup'
Here is a screenshot of this icon theme under Ubuntu Oneiric:
For more icon themes, check this page.