Trying to randomize stylesheet if a cookie has not been set

I've got a web site that allows the user to switch between two different CSS looks for the site via jQuery. Once a user selects one, it sets a 365 day cookie to remember which style sheet the user selected. I'm trying to figure out a way that if a cookie isn't set, to randomize between the two style sheets.

Here's the relevant jQuery (core and cookie plugin are loaded higher in the .js file). I'm think I need to add some sort of random "grab" in the else.

if($.cookie("css")) {
    $("link").attr("href",$.cookie("css"));
}else{

}
$(function() { 
    $("#designSwap li a").click(function() { 
        $("link").attr("href",$(this).attr('rel'));
        $.cookie("cs开发者_运维百科s",$(this).attr('rel'), {expires: 365, path: '/'});
        var themeColor = 'dark';
        updateColor($(this).attr('class'));
        return false;
    });
});

Here's the HTML:

<ul id="designSwap">
    <li><a class="dark" href="#" title="Alternate Design #1" rel="/lib/css/common-dark.css"></a></li>
    <li><a class="light" href="#" title="Default Design" rel="/lib/css/common.css"></a></li>
    <li>Choose your scheme:</li>
</ul>

Edit: I've got this up, running and functioning here: http://centerline.net, if anyone wants to see the swapping of style sheets and cookie setting in action.

var rand = Math.floor(Math.random()*2)
if (rand > 0)
   $("link").attr("href","/lib/css/common-dark.css");
else
   $("link").attr("href","/lib/css/common.css");