As Linux is such a modular operating system, and largely supported by open-source apps, there are countless ways to accomplish the same goals. Nevertheless, there are still a few very common programs used to alter the look and feel of the desktop. If you see something missing, feel free to add it!
1. Metacity: The standard window manager for Gnome, Metacity is a simple, usable window manager. It lacks many of the more advanced features some of the newer Window Managers support, but typically makes up for it with it’s increased speed.
2. GTK: The standard gnome widget manager, GTK essentially is responsible for the modification of the aspects of your desktop not controlled by a window manager. This includes buttons, scroll bars, and window interiors. Get GTK Themes here on customize.