Custom 404 on tumbl.com

gistfile1.txt

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script>
    /* Custom 404 */
    $(document).ready(function() {
        var page_404 = $("section.post h2 a:contains(Not Found)").length;
        if (page_404) {
            // you can do magic...
        }
    });
</script>