In Drupal 7, the Seven theme is the default administration theme. The administrative theme is used when performing all administrative tasks, most of which happen under the
/admin path. You can also choose to allow the administrative theme to be used when editing site content. Although some themes support Drupal’s administrative interface better than others, any Drupal theme can be used as an administration theme if desired.
The administration theme’s configuration settings are located below the theme listings on the
admin/appearance page. To use the same theme in both the front and back end of your Drupal site, simply choose Default theme as the Administration theme.