Returns HTML for a marker for new or updated content.
$variables: An associative array containing:
function theme_mark($variables) {
  $type = $variables['mark_type'];
  global $user;
  if ($user->uid) {
    if ($type == MARK_NEW) {
      return ' <span class="marker">' . t('new') . '</span>';
    }
    elseif ($type == MARK_UPDATED) {
      return ' <span class="marker">' . t('updated') . '</span>';
    }
  }
}