My main interest (and block) is that I need a category-specific header (it only displays large-font text and a navbar listing all five categories, with “home” replacing the category that is being viewed at the time.)

Content for each category is linear, much like a textbook is linear content flow, and all category content follows a chapter-logic which are sub and sub-sub etc. categories.

Any suggestion on how to do this in general, or how your code should be modified to accomplish this?

Really like your site, the design as well as your interdisciplinary content.