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 which allows content editors to create their own templates using themes without any HTML knowledge.
Themes are simple to create and manage. Themes are just a collection of CSS/JS files, images, and velocity files (either custom or using a standard framework), which are integrated into the dotCMS Theme Builder via references to specific classes and Velocity variables.
The dotCMS starter site comes with several pre-defined Themes based on the Bootstrap 4 framework, each of which includes template files which demonstrate how to integrate your own themes into dotCMS.