We needed to have an "internal" theme and a public facing "external" theme for all public pages, whether logged in or not. While civicrm_theme helped us theme our "internal" pages similarly to admin_theme, certain elements of CiviCRM our public facing and needed to be able to be defined with a different theme than that of the rest of CiviCRM. In our case, The CiviEvents needed to appear to the world in our "external" theme. Patch supplied to allow this to be defined.