I have a couple of questions:
- How do you change the desktop theme in CentOS?
- How does one download CentOS themes from the internet?
- How do you install them?
You can change your theme under this pull-down in CentOS:
System Settings > Advanced > Desktop Theme Details
Much of the theming infrastructure for GNOME2 is gone, for example: http://art.gnome.org/themes. But this site is still up and maintains icons, backgrounds, etc., for customizing your desktop.
NOTE: You'll want to focus on the GTK2.x content on sites as your encounter anything that is listed as GNOME-specific.