A quick update: Ambiance and Radiance Colors theme pack, which provides Ambiance and Radiance available in 8 different colors, is now available in a PPA for Ubuntu 11.10 Oneiric Ocelot and 12.04 Precise Pangolin users.
Using the PPA instead of manually installing the themes, you can stay up to date with newer versions without having to manually check for new updates. Add the PPA and install Ambiance and Radiance Colors GTK2/3 themes using the following commands:
sudo add-apt-repository ppa:ravefinity-project/ppa sudo apt-get update sudo apt-get install ambiance-colors radiance-colors
Once installed, use GNOME Tweak Tool or Ubuntu Tweak 0.6 to switch to any of the newly installed themes.
To install Ambiance and Radiance Colors for XFCE/LXDE, instead of the last command above, use:
sudo apt-get install ambiance-colors-lxxf radiance-colors-lxxf
The themes are of course also available on GNOME Look.