function get_randomCssNum()
{
    var ranCssNum= Math.floor(Math.random()*6);
    return ranCssNum;
}

function getaCss()
{
   var whichCss=get_randomCssNum();

    var cssName=new Array(6)
     cssName[0]="<link rel='stylesheet' type='text/css' href='/mt/css/bg1.css' />";
     cssName[1]="<link rel='stylesheet' type='text/css' href='/mt/css/bg2.css' />";
     cssName[2]="<link rel='stylesheet' type='text/css' href='/mt/css/bg3.css' />";
     cssName[3]="<link rel='stylesheet' type='text/css' href='/mt/css/bg4.css' />";
     cssName[4]="<link rel='stylesheet' type='text/css' href='/mt/css/bg5.css' />";
     cssName[5]="<link rel='stylesheet' type='text/css' href='/mt/css/bg6.css' />";

  	return cssName[whichCss]
  }
  document.write(getaCss());