function get_randomCssNum()
{
var ranCssNum= Math.floor(Math.random()*5);
//var ranCssNum= Math.floor(Math.random()*Number of CSS you Have);
return ranCssNum;
}

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

var cssName=new Array(5)

      // var cssName=new Array(Number of CSS you Have)
	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'>";
	cssName[4]="<link rel='stylesheet' type='text/css' href='css/e.css'>";

return cssName[whichCss]
}

document.write(getaCss());// JavaScript Document