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);     cssName[0]="<link rel='stylesheet' type='text/css' href='css/a.css'>";     cssName[1]="<link rel='stylesheet' type='text/css' href='css/b.css'>";     cssName[2]="<link rel='stylesheet' type='text/css' href='css/c.css'>";        cssName[3]="<link rel='stylesheet' type='text/css' href='css/d.css'>";

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