_includes/header.html

{% comment %} capture the current page {% endcomment %}
{% for entry in site.data.navigation %}
    {% capture fullurl %}{{ entry.url | absolute_url }}{% endcapture %}
    {% if fullurl == page.url %}
        {% assign current_page = fullurl %}
        {% break %}
    {% elsif page.url contains fullurl %}
        {% assign current_page = fullurl %}
    {% endif %}
{% endfor %}

<nav class="navbar navbar-expand-lg navbar-light bg-light sticky-top shadow-lg">
    <a class="navbar-brand" href="#">Navbar</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse" id="navbarSupportedContent">
        <div class="navbar-nav mr-auto" style="margin-left: 25%">

            {% for entry in site.data.navigation %}
                {% if entry.url == current_page %}
                    {% assign current = 'active' %}
                {% else %}
                    {% assign current = '' %}
                {% endif %}

                {% assign sublinks = entry.sublinks %}
                {% if sublinks %}
                    <li class="nav-item dropdown {{current}}">
                        <a class="nav-link dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false">
                            {{ entry.title }}
                        </a>
                        <div class="dropdown-menu">
                            {% for sublink in sublinks %}
                                <a class="dropdown-item" href="{{ sublink.url | absolute_url }}">
                                    {{ sublink.title }}
                                </a>
                            {% endfor %}
                        </div>
                    </li>
                {% else %}
                    <li class="nav-item {{current}}">
                        <a class="nav-link" href="{{ entry.url | absolute_url }}">
                            {{ entry.title }}
                        </a>
                    </li>
                {% endif %}
            {% endfor %}
        </ul>
    </div>
</nav>