Follow this. If you theme has a specific way of creating a child theme follow any of their own guides.
https://codex.wordpress.org/Child_Themes
The categories and topics are all weird. I just wish it looked standard like every other major forum out there.
It may be how the default category and forum layout bbPress uses. Maybe consider using this loop-forums.php file in a child theme.
https://github.com/robkk/bbPress-Hierarchical-Forum-and-Category-Layout