Add a special template for an individual page of a multipage webform

template.php

function THEME_preprocess_webform_form(&$vars) {
  /*
  Add a special template for an individual page of a webform.
  e.g. for the first page of a webform with a node id of 12: webform-form-12-0.tpl.php
  */
  $vars['theme_hook_suggestions'][] = $vars['theme_hook_suggestion'];
  $vars['theme_hook_suggestions'][] = 'webform_form_' . $vars['nid'] . '_' . $vars['form']['progressbar']['#page_num'];
  // The theme_hook_suggestion would override all suggestions so unset it.
  unset($vars['theme_hook_suggestion']);
}