styles = new Array(4);

styles[0] = "<link href='nil.css' rel='stylesheet' type='text/css'>";
styles[1] = "<link href='one.css' rel='stylesheet' type='text/css'>";
styles[2] = "<link href='two.css' rel='stylesheet' type='text/css'>";
styles[3] = "<link href='three.css' rel='stylesheet' type='text/css'>";
index = Math.floor(Math.random() * styles.length);

document.write(styles[index]);