And those words are used many, many times in the text you need to customize. It’s a pretty terrible idea to change them. Changing Topic to Post is an especially bad idea, since both of those words already have meanings and you’ll quickly never know which meaning you are talking about. If you decide to do this you’ll likely be very sorry you did.
Renaming Categories to “chapters” or whatever else is probably fine. Of course, you’re not going to be able to change the path, so if Categories become “rooms” the url will still be /c/slug/123.
Changing “topic” to “thread” used to be less bad of an idea, but now we have th notion of “thread” in chats, so it’s very confusing that way too.