EDIT:
@baris error is gone / fixed. Thank you!
Nodebb is really nice forum software. Unfortunately there are hardly any themes for it, so you have to help yourself. The developers of nodebb have integrated css, html, javascript so that you can edit the header use yourself. What I fundamentally miss in this software, however, is that you can not only design the header with html but also the footer. If there was another tab just for the footer, you could easily adapt the forum to your own website design. So unfortunately only the possibility via iframe remains, which is an ugly solution
maybe that can still be integrated on your part
I thank you @phenomlab. Without you I would be really at a loss sometimes
Do you have any ideas how I can get the top menu down?