Integrate the book well

Currently, there's an iframe integration of the book into the page (currently not even the right book).

That does work surprisingly well, but it would be great if the book had a little bit of a theme.

The only necessary feature is that the header of the page stays visible.

Start by locating the page's current iframe integration and the embedded book content. Check how the page header is rendered and determine how a theme can be applied while keeping that header visible; done means the correct book is integrated with a theme and the header remains visible.

Written by the indexing model from the issue text.