function rand(){
    return Math.round(Math.random()*(css.length-1));
}
var css = new Array(
'<link rel="stylesheet" type="text/css" href="default.css">',
'<link rel="stylesheet" type="text/css" href="style2.css">',
'<link rel="stylesheet" type="text/css" href="style3.css">',
'<link rel="stylesheet" type="text/css" href="style4.css">'
);
rand = rand();
document.write(css[rand]);