actually add template

_includes/postlistbytopic.html

+<div>
+  <div  style="background-color: rgb(55,55,55);">
+    <h2 style="margin: 5px;">Weitere Artikel zu diesem Thema</h2>
+  </div>
+<ul style="margin: 25px;">
+  {% for post in site.posts reversed %}
+    {% if post.tags contains {{include.topic}} %}
+    <li>
+      <a href="{{ site.baseurl }}{{ post.url }}">{{post.title}}</a>
+    </li>
+    {% endif %}
+  {% endfor %}
+</ul>
+</div>