/**
 * Implements hook_theme().
 */
function mymodule_theme() {
  return array(
    'page_node_form' => array(
      'render element' => 'form'
    ),
  );
}

/**
 * Revised output for the form.
 *
 * @param $variables
 *   An associative array containing:
 *   - form: A render element representing the form.
 *
 * @ingroup themeable
 */
function theme_page_node_form(&$variables) {
  $form = $variables['form'];
  // Blah blah blah...
  return drupal_render($form);
}