12.04 gnome classic ambiance font problem
The bug is reported https://bugs.launchpad.net/ubuntu/+source/light-themes/+bug/981289 and a fix is available in a branch on Launchpad, but it hasn't been merged yet, so it's not available in the official Ubuntu 12.04 repositories.
A solution is to use the fixed "light-themes" (Ambiance and Radiance) package available in the WebUpd8 themes PPA. Add the PPA and upgrade to get the package using the commands below:
sudo add-apt-repository ppa:webupd8team/themes
sudo apt-get update
sudo apt-get upgrade
Reference