Resolve "Learning materials"

Review here

Closes #276 (closed) #236 (closed)

_includes/header.html

@@ -64,7 +64,7 @@ See also:
                         {% for entry in category.entries %}
                         <ul>
                         {% if entry.entries %}
-                            <li>
+                            <li class="nav-second-level">
                                 <details>
                                     <summary>
                                         {{ entry.title }}
@@ -81,7 +81,7 @@ See also:
                                 </details>
                             </li>
                         {% else %}
-                            <li>
+                            <li class="nav-second-level">
                                 <a href="{{ entry.url | relative_url }}">
                                     {{ entry.title }} {% if entry.external %}<i class="fas fa-external-link-alt"></i>{% endif %}
                                 </a>