function get_randomCssNum()
{
    var ranCssNum= Math.floor(Math.random()*4);
	//var ranCssNum= Math.floor(Math.random()*Number of CSS you Have);
    return ranCssNum;
}

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

    var cssName=new Array(4)
     // var cssName=new Array(Number of CSS you Have)
     cssName[0]="<link rel='stylesheet' type='text/css' href='0a1.css'>";
     cssName[1]="<link rel='stylesheet' type='text/css' href='0a2.css'>";
     cssName[2]="<link rel='stylesheet' type='text/css' href='0a3.css'>";
     cssName[3]="<link rel='stylesheet' type='text/css' href='0a4.css'>"; 
         
     //I am using 4 CSS files in this example. you can add or remove ;)
     //for Add more CSS just add line like this 
     //cssName[Next Number]="<link rel='stylesheet' type='text/css' href='CSS FILE URL'>";
  	return cssName[whichCss]
  }
  document.write(getaCss());