Adding CSS to an entire book

I currently am working on a HackMD book to explain a lot of Android-Modding related things for a telegram chat I have, and _**I am somewhat frustrated by the fact that `` tags won't automatically get loaded for any of the categories if I add it to the main book's note.**_ (the note which defines categories on the left hand side)

Maybe we should load `<style>` tags for any sub-notes instead of having it load as `&lt;style&gt;` in page source, which obviously won't get recognised as a style tag.

One might argue: "just add the CSS to every sub-note!"
though that is quite unpractical when you've only vaguely touched a topic and you're already at like 7 categories and a bunch of notes in those categories.