addons-server/templates/includes/refinement.html

<li {{ item.selected|class_selected(True) }}>
  <a href="{{ item.url }}">{{ item.text }}</a>
</li>
{% if item.children %}
  <li>
    <ul>
      {% for child in item.children %}
      <li{% if child.selected %} class="selected"{% endif %}>
        <a href="{{ child.url }}">{{ child.text }}</a>
      </li>
      {% endfor %}
    </ul>
  </li>
{% endif %}