netbox/netbox/templates/ui/panels/_base.html

<!-- begin {{ panel_class|default:"panel" }} -->
<div class="card">
  {% if title or actions %}
    <h2 class="card-header">
      {{ title|default:"" }}
      {% if actions %}
        <div class="card-actions">
          {% for action in actions %}
            {% render action %}
          {% endfor %}
        </div>
      {% endif %}
    </h2>
  {% endif %}
  {% block panel_content %}{% endblock %}
</div>
<!-- end {{ panel_class|default:"panel" }} -->