How can I access this site as the administrator?
I think default is substituted for the site name in your drush result, try http://example.com/user/reset/1 which would be the standard password reset page for a user with a uid of 1 (the admin user).

If that doesn't work the default theme is set by the theme_default variable...you can edit the variable table in the database manually and set the value column for that variable to whatever theme you want.