var ArrayCSS = new Array("../css/style_1.css", "../css/style_2.css", "../css/style_3.css", "../css/style_4.css");

while (true)
{
   var num = Math.round((Math.random() * 10));
   
   if (num >= 0 && num <= (ArrayCSS.length-1))
   {
      document.write('<link href="' + ArrayCSS[num] + '" rel="stylesheet" type="text/css" media="screen" />');
      break;
   }
}