popsu-d7/sites/all/modules/admin/includes/admin.theme.inc

<?php

/**
 * Theme switcher admin block.
 */
function admin_block_theme() {
  $themes = list_themes();
  if (count($themes) > 2) {
    return array(
      'subject' => t('Switch theme'),
      'content' => drupal_get_form('admin_block_theme_form', $themes),
    );
  }
}

/**
 * Devel admin block form.
 */
function admin_block_theme_form($form, $form_state, $themes) {
  $options = array();
  foreach ($themes as $theme) {
    if ($theme->status) {
      $options[$theme->name] = isset($theme->info['name']) ? check_plain($theme->info['name']) : $theme->name;
    }
  }
  $form = array();
  $form['theme_default'] = array(
    '#type' => 'radios',
    '#options' => $options,
    '#default_value' => variable_get('theme_default', 'garland'),
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save configuration'),
  );
  $form['#submit'] = array('system_settings_form_submit');
  return $form;
}