Allows switching theme via $_GET parameter.

mymodule.module

<?php

/**
 * Implements hook_custom_theme().
 */
function mymodule_products_custom_theme() {
  $themes = list_themes();

  if (isset($_GET['theme']) && $_GET['theme'] && isset($themes[$_GET['theme']]) && $themes[$_GET['theme']]->status) {
   return $_GET['theme'];
  }
}