A lot works!

templates/main/nav.html

@@ -1,5 +1,5 @@
 <nav class="navbar navbar-expand-md navbar-light bg-light fixed-top">
-    <a class="navbar-brand" href="{{ url_for("index") }}">Marlous</a>
+    <a class="navbar-brand font-weight-bold" href="{{ url_for("index") }}">Marlous</a>
 
     <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav"
             aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
@@ -8,8 +8,19 @@
 
     <div class="collapse navbar-collapse" id="navbarNav">
         <ul class="navbar-nav">
-            <li class="nav-item active">
-                <a class="nav-link" href="#">Home<span class="sr-only">(current)</span></a>
+            {% set items = [("Product Design", 'product_design'), ("Grafisch", "grafisch"), ("Fotografie", "fotografie")] %}
+            {% for item in items %}
+                <li class="nav-item">
+                    <a class="nav-link {% if item[1] == request.path[1:] %}active{% endif %}"
+                       href={{ url_for("category", category_name=item[1]) }}>
+                        {{ item[0] }}
+                    </a>
+                </li>
+            {% endfor %}
+
+            <li class="nav-item">
+                {% set name = "contact" %}
+                <a class="nav-link {% if name == request.path[1:] %}active{% endif %}" href="{{ url_for("contact") }}">Contact</a>
             </li>
         </ul>
     </div>