There must be a conflict with Avada and the more or less new layout-methods. On pages, where I have nothing with wp-foro, the layouts get loaded normally. But as soon as I open a forum, all the header styles disapear.
Did anyone here experience the same issue?
I am sorry. I have read again through the documentation and I have found this: https://wpforo.com/docs/root/getting-started/forum-page/turn-wordpress-to-wpforo/
So I unchecked the checkbox and now it works.