MonoBlack is a set of black icons that can best fit with bright and white GTK3 themes. In this tutorial we will see how to install it under Ubuntu 11.10 Oneiric Ocelot, you can also test it under Ubuntu 12.04 (Alpha).
Open the terminal and run the following commands:
wget -O MonoBlack.tar.gz http://goo.gl/cjfsu
sudo tar -xzvf MonoBlack.tar.gz -C /usr/share/icons
Enable now this icon theme with this command:
gsettings set org.gnome.desktop.interface icon-theme 'MonoBlack'
A screenshot of it under Ubuntu Oneiric:
You can find here more icon themes.