I'm trying to change the Ebony warrior outfit into light armor, but that whole section is greyed out... why?

He is probably using a template and is inheriting the inventory from it. This overrides the ability to make direct edits to the actor. What you can do is simply uncheck the box. You can't miss it, as all the flags are under the Add Script field.

More info: http://www.creationkit.com/Actor