Themes allow the user to change the look and feel of a website. Many content management systems, e.g. XOOPS, Moodles, Joomla!, or Drupal, have similar theming systems that place the theme resources (i.e. css and images) inside a folder. Changing themes is simply the same as changing the folder to load the stylesheets and images. Extending an existing theme could be done by first making a copy of the original theme, and then modify the css style rules as necessary.