The way afuna explained it to me is this:

Copy/paste TB's code into your layout layer, not only the stylesheet but the whole code. Just c/p really. Then add your own CSS in print_stylesheet.

It may help you to look at the code of one of the other TB-based layouts: Modish for example. See how the first's part is just the Tabula Rasa code? Then you have a second part for the Modish layout.