Litalino unfortunately that Nightmode issue has been a pain and I have no clear idea how to fix it because Nightmode causes such a deep impact on the Flarum assets and frontend logic. It's really a Nightmode issue since it also breaks other extensions that use custom fronteneds. At this time I don't have any plan to fix it (since I have absolutely no idea how...)
CyberGene I have not tried, but something as simple as .Header-controls .item-flamarkt-backoffice { display: none } should do it. .Header-controls would not even be required but it's a bit safer so it doesn't start hiding stuff in other unexpected places.
