Hi joos,

Please read the wpForo documentation. it's very easy:

https://wpforo.com/docs/root/categories-and-forums/forum-layouts/

Please read the tips of the Forums admin page.

Forums can be displayed with different layouts (Extended, Simplified, Q&A), just edit the top (blue panels) category and set the layout you want. Child forums' layout depends on the top category (blue panels) layout. They cannot have a different layout.