How do I theme the "The website encountered an unexpected error" page?
The errors in Drupal 7 handled by includes/errors.inc file, and the The website encountered an unexpected error. Please try again later. error use the maintenance_page theme see from includes/errors.inc  

print theme('maintenance_page', array(
        'content' =&gt; t('The website encountered an unexpected error. Please try again later.'),
      ));
      exit;


So if you want to theme the error page, you theme the maintenance_page to do so take a look at Theming the Drupal maintenance page.