theme-settings.php

<?php
/**
 * Implements hook_form_system_theme_settings_alter() function.
 *
 * @param $form
 *   Nested array of form elements that comprise the form.
 * @param $form_state
 *   A keyed array containing the current state of the form.
 */
function voltron_form_system_theme_settings_alter(&$form, $form_state, $form_id = NULL) {
  // Work-around for a core bug affecting admin themes. See issue #943212.
  if (isset($form_id)) {
    return;
  }

  // Create the form using Forms API
  $form['themedev']['voltron_rebuild_registry'] = array(
    '#type'          => 'checkbox',
    '#title'         => t('Rebuild theme registry on every page.'),
    '#default_value' => theme_get_setting('voltron_rebuild_registry'),
    '#description'   => t('During theme development, it can be very useful to continuously <a href="!link">rebuild the theme registry</a>. WARNING: this is a huge performance penalty and must be turned off on production websites.', array('!link' => 'http://drupal.org/node/173880#theme-registry')),
  );
}