I researched al bit further and I noticed my question may be wrong.
bp-parent-forum-### is not the right one, I need the most parent one.
I want to add custom CSS to all subforums / topics and posts of the upper parent forum. Preferably to the <body> element because then I can then edit the whole page. The class could have the id of the most parent forum or a custom custom added class (maybe with a custom field?) that is not added to other upper parent forums.
What I’m trying to resolve is the following. My client wants forums in multiple languages, one of them is in Arabic. Text needs to be aligned to the right, but only for all forums and posts belonging that part.