A <em>theme</em> is a set of files that define the visual look and feel of your site. The core software and modules that run on your site determine which content (including HTML text and other data stored in the database, uploaded images, and any other asset files) is displayed on the pages of your site. The theme determines the HTML markup and CSS styling that wraps the content. Several basic themes are supplied with the core software; additional <em>contributed themes</em> can be downloaded separately from the <a href="https://www.drupal.org/project/project_theme">Download &amp; Extend page on drupal.org</a>, or you can create your own theme.