Your system comes with many optional themes; however, you may also download more from many sources online. To do this:
Visit a website that lists downloadable desktop elements such as themes, wallpapers, and icons for your desktop. One excellent source is www.gnome-look.org.
Download the theme you have selected and save it on your computer. It may come in a .zip file or a tar.gz file. If it comes in a zip file, extract the contents until you come to a tar.gz file. If you download a tar.gz file, leave it as is.
To install the theme, go to
Navigate to the location of the downloaded theme.
Select the downloaded theme and click
To use the theme, simply select it and the changes will be reflected on your desktop automatically.