¿Solo categoría de enlaces?

[Falco]

How can I customise CSS for each category? (as in having different design for each category)

When you’re in a category, discourse adds the category slug to the body element. For example, on meta here when I visit the bug category I get category-bug in the body tag. If you use that as a selector you can style each category differently.

But since people can post to one category while browsing another, it won’t work as you want to.