Bootscrap
I'm sure something like this already exists, but I couln't find it quickly so I made (another?) one myself.
This is basically an NPM package with the express purpose of providing programmatic access to Bootstrap and Bootswatch CSS files.
Usage
var bs = ; bs // gets the CSS for the default, minified bootstrap.min.cssbs // gets the CSS for the 'Flatly' Bootswatch themebs // etc., etc.
Knock yourselves out.