Not strictly CSS, but if you enable the Free Nodelet under Nodelet Settings, and paste the following into your Free Nodelet Settings, then you'll get syntax highlighting for all code samples posted on the site:

<link rel="stylesheet" href="http://buzzword.org.uk/2012/codemirror-2.
+36/lib/codemirror.css" type="text/css" media="all" />
<script src="http://code.jquery.com/jquery-1.8.3.min.js" type="text/ja
+vascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/lib/codemirro
+r.js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/perl/per
+l.js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/xml/xml.
+js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/css/css.
+js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/javascri
+pt/javascript.js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/htmlmixe
+d/htmlmixed.js" type="text/javascript"></script>
<script type="text/javascript">
$(function() {
    $farabi = $('<form action="http://localhost:3000/" method="post" s
+tyle="display:none"><textarea name="source"></form>')
    $('body').append($farabi);
    $('p.code').each(function (i, c) {
        var $code = $(c).find('tt.codetext').text().replace(/\u00a0/g,
+' ').replace(/\s+$/,'');
        var $dl   = $(c).find('span.embed-code-dl');
        
        var $pre = $(
            '<textarea class="CodeMirror">'
            + $('<div/>').text($code).html()
            + '</textarea>'
        );
        var $footer = $('<div style="text-align:right"><small></small>
+</div>');
        var $editlink = $('<a href="#">farabi</a>');
        $editlink.click(function () {
            $farabi.find('textarea').val($code);
            $farabi.submit();
            return false;
        });
        $footer.find('small').append("&#x5B;");
        $footer.find('small').append($editlink);
        $footer.find('small').append("&#x5D; ");
        if ($dl.html()) $footer.find('small').append($dl.html());
        $(c).after($footer);
        $(c).replaceWith($pre);
        
        CodeMirror.fromTextArea($pre.get(0), {
            lineNumbers: true,
            readOnly: true,
            matchBrackets: true,
            lineWrapping: true
        });
    });
});
</script>
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