Returns HTML for an element's children fieldsets as vertical tabs.
$variables: An associative array containing:
function theme_vertical_tabs($variables) {
$element = $variables['element'];
// Add required JavaScript and Stylesheet.
drupal_add_library('system', 'drupal.vertical-tabs');
$output = '<h2 class="element-invisible">' . t('Vertical Tabs') . '</h2>';
$output .= '<div class="vertical-tabs-panes">' . $element['#children'] . '</div>';
return $output;
}