theme.html

<!-- Add the following lines to theme's html code right before </head> -->
<link rel="stylesheet" href="//gist.github.com/assets/embed-bfcadee384027982121fdeb9930e78b2.css" />

<!-- add the folowing if jquery isn't already included in your theme-->
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.1/jquery.min.js"></script>
<!-- add the following to the bottom of the theme-->
<script>
    $(function() {
        function makeGist(d){
            var link = $($(d).html())[0];
            if(link){
                $.ajax({
                    url:link.innerHTML+'.json',
                    dataType:'jsonp'
                }).then(function(data){
                    d.html(data.div);
                });
            }
        }
        $('.gist').each(function(i,v){
            makeGist($(v));
        });
    });
</script>
<!--
  Usage: just add <div class="gist">[gist URL]</div>
  Example: <div class="gist">https://gist.github.com/1395926</div>
-->

Thanks!!