SO CSS styles won't refresh after AJAX runs

http://stackoverflow.com/a/18648985/1370556

HTML

<div id="index">
    <div id="contentcontainer"></div>
    <button id="appendBtn" type="button">Append</button>
</div>

CSS

#index {
    background-color:#82e4e5;
}
#portfolio {
    background-color:#fb5656;
}

JavaScript

// Not sure where you're getting divID from
var divID = "portfolio";

$("#appendBtn").click(function (e) {
    $.ajax({
        //type: "GET",
        //url: pageToLoad,
        type: 'post', // For Ajax Echo
        url: "/echo/html/", // For Ajax Echo
        async: false, // Do you really need this?
        // For Ajax Echo - NOTE THE SURROUNDING DIV
        data: {
            html: '<div><div class="content" id="portfolio"><div class="wrapper"><h2>Portfolio</h2><p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Cras vestibulum libero non egestas dapibus. Nam gravida, libero ac posuere eleifend, mauris nisi venenatis metus, non hendrerit magna justo eget lectus.</p></div><!--wrapper--><div class="clear"></div></div><!--content--></div>'
        },
        success: function (data) {
            // Remove trigger and use a better selection format
            $(data).appendTo('#contentcontainer');
        }
    });
});