Hard to say. Your terminology here is quite confusing.
Going back to the Meta example. The top level, meta.discourse.org, is the “site”, or “forum”. Within Meta there are different “categories”, like support, bug, feature, etc. Within each category are “topics”, like this one we’re interacting in. Each of our replies within this topic are “posts”.
Yes, you can absolutely customize each categories appearance separately via CSS. A good example of this is https://forums.funcom.com. They use a different category for each of their games. Notice the different appearance of the Conan Exiles category than the Secret World Legends category.