function get_randomCssNum()
{
    var ranCssNum= Math.floor(Math.random()*3);
	//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(3)
     // var cssName=new Array(Number of <strong class="highlight">CSS</strong> you Have)
     cssName[0]="<link rel='stylesheet' type='text/css' href='/sites/carterroofingcomau/assets/css/bg01.css'>";
     cssName[1]="<link rel='stylesheet' type='text/css' href='/sites/carterroofingcomau/assets/css/bg02.css'>";
     cssName[2]="<link rel='stylesheet' type='text/css' href='/sites/carterroofingcomau/assets/css/bg03.css'>";   
     //cssName[3]="<link rel='stylesheet' type='text/css' href='d.css'>";
     //cssName[4]="<link rel='stylesheet' type='text/css' href='e.css'>";
     //I am using 5 <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='CSS FILE URL'>";
  	return cssName[whichCss]
}

document.write( getaCss() );