You should be able to grab the CSS out of there and set include it in your Docverter call with other_files[]=@filename and css=filename, assuming you're pushing Markdown. If you're pushing HTML just drop the styles you want directly in the header.
I used a bit of the RailsApp when I was learning Rails to build a subscription site. Luckily I met Pete and was able to use his book when I actually got to the subscription impl. I KNOW my app as a whole is shitty, so I just bought yours to see how I can unfuck ^^
(my latest ebook = http://gilesbowkett.blogspot.com/2013/10/new-ebook-software-...)