jekyll/docs/_includes/primary-nav-items.html

<ul>
  {% for p in site.data.primary_nav %}
  <li class="
    {% if p.link == '/' %}
      {% if page.url == '/' %}current{% endif %}
    {% else %}
      {% if page.url contains p.link %}current{% endif %}
    {% endif %}">
    <a href="{{ p.link }}">{{ p.title }}</a>
  </li>
  {% endfor %}
</ul>