A theme controls the visual presentation of a Drupal site. It defines the templates, stylesheets, JavaScript, and assets that turn raw content into what visitors see in their browsers.
Drupal themes are made up of Twig template files, YAML configuration, CSS, and sometimes preprocess functions written in PHP. You can build a theme from scratch or start from a base theme like Olivero, Claro, or increasingly Gin.
Themes are separate from content and configuration, which means you can completely redesign a site without touching its data. This separation is one of Drupal’s strengths. For most projects, a custom theme built on a solid base gives you the best mix of flexibility and maintainability.