Gnome is one of the most used windows manager for Linux, it is also highly configurable.
You can customize it by changing themes, or on each theme changing the controls, windows borders or icons.
First thing to do is to download some good themes.
You can use Google or use one of these sites I have found.
Once you have downloaded the file, you need to go to menu:
Desktop -> Preferences -> Theme for Debian, and I think
Desktop -> Preferences -> Appearance for Distros with newer Gnome version, like Gutsy.
For The older Gnome you will see this.
Now you can drag and drop the themes files directly to that window, and you will have your new themes installed.
If what you want to install is a specific part of a theme, like windows borders or icons, you have to click on “Theme details” button, and you will see something like this.
There you need to choose Controls, Window Border or Icons, depending on what you what you want to install, and then drag and drop the file to the window, or you can also drag the link there and it will also work (If you are connected to the Internet of course).
Once installed you can customize your Desktop by selecting different icons, window borders, for any Theme and mixing them, and therefore beautify your Linux