forum: reply page, CSS, and first_unread fix

forum/templates/forum/topic.jinja

@@ -39,6 +39,9 @@
     <p><a href="{{ url('forum:new_message', topic_id=topic.id) }}">{% trans %}Reply{% endtrans %}</a></p>
 
     {% for m in topic.messages.select_related('author__avatar_pict').all() %}
+        {% if m.id == first_unread_message_id %}
+        <span id="first_unread"></span>
+        {% endif %}
         {% if m.id >= first_unread_message_id %}
         {{ display_message(m, user, True) }}
         {% else %}