This can range from the background images you see, right down to the sound that plays when you open a new window. Themes give your computer's desktop a unique 'atmosphere'. For example, if you're writing some code, then you may want a simple theme that doesn't distract the eye. If you're chatting on IRC or casually browsing the web, then a theme with bright, colourful borders and a fancy background image might be more suitable. You can download a theme from Themes.org or design your own.
Themes vary from window manager to window manager.
Some window managers provide the facility to easily create your own themes by using theming tools while others require scripting skills and artistic talent. Themes.org is the central resource for themes. No matter what your personal tastes, or current mood is you will find a theme for it on Themes.org. So, take a few minutes and wander around our many theme galleries. If you have created your own theme which you'd like to add to themes.org then simply click on the icon above that matches the window manager that your theme is for.