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if(isset($_GET['theme'])) {
   $mytheme = $_GET['theme'];
   setcookie("SessionTheme", $mytheme);
} else {
   if (isset($_COOKIE['SessionTheme'])) {
      $mytheme = $_COOKIE['SessionTheme'];
   } else {
      $mytheme = 'themenamehere';
   }
}
SSViewer::set_theme($mytheme);