Find out which theme is loaded in a given page
In Drupal 8, you can do two things from custom code:

\Drupal::theme()-&gt;getActiveTheme();

\Drupal::theme()-&gt;getActiveTheme()-&gt;getName();


The first will return a theme object, the second will return just the theme name.

See the ThemeManager docs for more details.

You can also install Devel/WebProfiler for more information about the current page request and other variables.