function rand(){
    return Math.round(Math.random()*(css.length-1));
}

var css = new Array(
	'<link rel="stylesheet" type="text/css" href="css/dd_bg1.css">',
	'<link rel="stylesheet" type="text/css" href="css/dd_bg2.css">',
	'<link rel="stylesheet" type="text/css" href="css/dd_bg3.css">'
);

rand = rand();

document.write(css[rand]);