The themes directory has an _includes subdirectory. It has a few helper files.
_includes/bootstrap/head.pug
This header file includes the DOCTYPE and head of a page.
It sets the title approriately and links to
theme/assets/bootstrap.min.css and
theme/assets/font-awesome/css/font-awesome.min.css. It assumes that those
files exist in any theme using them. See some of the bootstrap themes for
examples.
_includes/bootstrap/script.pug
This file references all the JS files needed for Bootstrap to work. It assumes that those files exist in any theme using them. See some of the bootstrap themes for examples.