function get_randomCssNum()
{
    var ranCssNum= Math.floor(Math.random()*4);
	//var ranCssNum= Math.floor(Math.random()*Number of <strong class="highlight">CSS</strong> you Have);
    return ranCssNum;
}

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

    var cssName=new Array(4)
     // var cssName=new Array(Number of <strong class="highlight">CSS</strong> you Have)
     cssName[0]="<link rel='stylesheet' type='text/css' href='css/1.css'>";
     cssName[1]="<link rel='stylesheet' type='text/css' href='css/2.css'>";
     cssName[2]="<link rel='stylesheet' type='text/css' href='css/3.css'>";   
     cssName[3]="<link rel='stylesheet' type='text/css' href='css/4.css'>";
     //I am using 4 <strong class="highlight">CSS</strong> files in this example. you can add or remove ;)
     //for Add more <strong class="highlight">CSS</strong> just add line like this 
     //cssName[Next Number]="<link rel='stylesheet' type='text/css' href='<strong class="highlight">CSS</strong> <strong class="highlight">FILE</strong> URL'>";
  	return cssName[whichCss]
  }
document.write(getaCss());