CentOS 6 is great OS for desktop use but the default Gnome installation doesn’t look so good and the available themes are not of much help. I find the Nimbus engine (the old OpenSolaris GTK theme) to be a great choice to revive the old Gnome 2.
The easiest way to install it on Centos 6 / Scientific Linux 6 / Red Hat Enterprise Linux 6 and / or Oracle Linux 6 is by use of the RPMForge repository which is compatible with all RHEL clones.
yum install http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el6.rf.x86_64.rpm
yum install gtk-nimbus-engine
After the installation you can easily enable it through System –> Preferences –> Appearance.