Our template installs a default theme (as a Ruby gem) called ‘Classic’. This theme includes a set of stylesheets for four output formats: the web (web.css), ebooks (epub.css), print PDF (print-pdf.css) and screen PDF (screen-pdf.css).

You can override anything in the default theme:

  1. by editing the template stylesheets for each book in its styles folder;
  2. by adding your own alternative files to _includes and _layouts folders.

To see the parent styles and files you’re overriding, browse the Classic theme repo.

For more detail, see the official Jekyll guidance on themes.