How to get 'base theme' information about a theme?
You can get the details of a theme by using ThemeHandler::listInfo().

$child_theme = 'bartik';

$base_theme = \Drupal::service('theme_handler')-&gt;listInfo()[$child_theme]-&gt;base_theme;

$base_themes = \Drupal::service('theme_handler')-&gt;listInfo()[$child_theme]-&gt;base_themes;