If you would like to have Horizontal Menu (or Topnav) based layout then perform below change.

Each of the layout options is provided below with steps you would need to perform in file Admin/templates/partials/base.html and add data-layout="horizontal" in body tag.

To change Horizontal layout header, comment vertical block header & block sidebar as shown below.

    {% comment %}
        {% block header %}
            {% include 'partials/header.html' %}
        {% endblock %}

        {% block sidebar %}
            {% include 'partials/sidebar.html' %}
        {% endblock %}
    {% endcomment %}

and un-comment block header as shown below.

{% block header %}
{% include 'partials/hori-header.html' %}
{% endblock %}

{% block sidebar %}
{% include 'partials/hori-sidebar.html' %}
{% endblock %}