In this tutorial we will see how to install and enable the Amora icon theme under Ubuntu 11.10 Oneiric Ocelot. You should know that this theme is not complete and is best suited for dark themes.
Open the terminal and run the following commands:
sudo unzip Amora.zip -d /usr/share/icons/
To enable Amora, use Gnome Tweak Tool or run simply this command:
gsettings set org.gnome.desktop.interface icon-theme 'Amora'
Here is a screenshot of it under Ubuntu Oneiric:
For more icon themes, check this list.