var cssnumber = Math.floor(Math.random()*3+1);  
  if (cssnumber == 1)
      display = "css/top1.css"; 
  else if (cssnumber == 2)
      display = "css/top2.css"; 
  else if (cssnumber == 3)
      display = "css/top3.css"; 
  var css = '<'; 
  css+='link rel="stylesheet" type="text/css" href=' + display + '  \/'; 
  css+='>'; 
document.write(css);