function get_randomCssNum()
{
    var ranCssNum= Math.floor(Math.random()*3);
	//var ranCssNum= Math.floor(Math.random()*Number of CSS you Have);
    return ranCssNum;
}

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

    var cssName=new Array(3)
     // var cssName=new Array(Number of CSS you Have)
     cssName[0]="<link rel='stylesheet' type='text/css' href='/wp-content/themes/ts/style.css'>";
     cssName[1]="<link rel='stylesheet' type='text/css' href='/wp-content/themes/ts/style2.css'>";
     cssName[2]="<link rel='stylesheet' type='text/css' href='/wp-content/themes/ts/style3.css'>";   
     //I am using 5 CSS files in this example. you can add or remove ;)
     //for Add more CSS just add line like this 
     //cssName[Next Number]="<link rel='<strong class="highlight">stylesheet</strong>' type='text/css' href='CSS FILE URL'>";
  	return cssName[whichCss]
  }
  document.write(getaCss());