var cssnumber = Math.floor(Math.random()*4+1); 
  if (cssnumber == 1)
      display = "css/main01.css";
  else if (cssnumber == 2)
      display = "css/main02.css";
  else if (cssnumber == 3)
      display = "css/main03.css";
  else if (cssnumber == 4)
      display = "css/main01.css";
  else
      display = "css/main01.css";
  var css = '<';
  css+='link rel="stylesheet" href=' + display + ' \/';
  css+='>';
document.write(css);