Go to Appearance in the admin menu. At the bottom of the page you will see a section called Administration theme with a dropdown listing available admin themes.
By default, Drupal 11 ships with Claro as the admin theme. You can switch to any installed theme, though themes designed specifically for the admin area give the best experience. Gin is increasingly popular as an alternative and is being brought into core for Drupal 12.
To install a new admin theme, add it with Composer, then set it as the administration theme on the Appearance page. Changing the admin theme affects only logged in users with permission to use the admin interface. Visitor facing pages continue to use the default public theme.