Thanks for your reply. The problem is that Book Lite supports custom headers but only displays the custom header for pages, not for full-page single posts. I can't work out where in the theme files this behavior is specified, or where in the CSS some other minor differences stem from.
For example, in the demo, the main page has a custom header but this single post does not. Also note that the <h1> within the header of the main page has a max-width of 750px, and the header of the single post does not.
I can't find where in the theme files these differences stem from. I want edit the php to make every page and post share a single header which I can then style in CSS.