dotCMS Themes provide a powerful way to abstract and share a consistant look and feel across a site or sites. Themes work with the the Template Designer that allows content editors to create their own templates using themes without any HTML knowledge.
Themes are simple to create and manage - they are just a collection of CSS/JS files, images and velocity files.
You can either create your own theme or download and use an existing theme. dotCMS has a Theme Library where you can download themes for use in your dotCMS installation.