What are Themes?
 

Themes are a collection of CSS files, .skin files, and images. They are text based style definitions and are very similar to CSS, in that they provide a common look and feel throughout the website.

A theme is a combination of pictures, colors, and sounds on your computer. It includes a desktop background, a screen saver, a window border color, and a sound scheme

its sort of wallpaper

you should search on google. more information...@@