Themes in Drupal are responsible for the look and feel of your site. They provide the HTML markup, CSS, typography, images, and interactive experience that people encounter when they view your site. Each theme is a set of files including style sheets, Twig templates, Drupal-specific YAML files, images, and more, all contained within a directory. There are hundreds of community-contributed themes available. Themes are often one of the first places you'll start to modify the code and make your Drupal site truly unique.

Developers can create new themes to customize the look and feel of a site. This can be done by either extending an existing theme and making minor changes, or creating a completely custom design.

There are three different types of themes:

• core themes: themes that are included with Drupal core and live in the /core/themes directory
• contributed themes: themes that are download from Drupal.org, and are typically placed in the /themes/contrib directory
• custom themes: themes that you or a theme developer created specifically for your use case, and are typically placed in the /themes/custom directory