indico-plugins/previewer_code/indico_previewer_code/templates/pygments_preview.html

<style>

    {{ css_code }}

    .highlight pre {
        font-size: 0.9em;
    }

    .lineno {
        color: #aaa;
        padding-right: 0.5em;
    }

</style>

<pre class="text-preview-content">
    {{ html_code | safe }}
</pre>