Configure your theme to use another theme as the base theme.

Determine the theme's name

Find the machine name of the theme you would like to use as a base theme.

Edit your .info.yml file

Edit your theme's THEMENAME.info.yml file, add a base themekey to the existing YAML metadata.

Clear the cache

Clear Drupal's internal cache using either the Drupal UI, or a tool like drush, so that the changes to your .info.yml file are read by Drupal.

Take a look at this video: