if (isset($_COOKIE["css"])) {
    $style = $_COOKIE['css'];
} else {
  $style = $_GET['css'];
  setcookie("css", $style, time()+3600*96);
}

switch($_GET['css'])
{
        case 'style2':
         $style = '/styles/second.css';
        break;
        default:
        case 'style1':
         $style = '/styles/default.css';
        break;