If you'll take a look at
src/vendors/bootstrap/bootstrap.scss, you'll see all the files that are imported into the theme. The
~ in front of the path means that the file is imported from
node_modules. The cards, responsive-embed and mixins are all included. I'm not sure if the version of the bootstrap package influences things in any way.
You can proceed and import your own file inside of this one and it will be processed by gulp.
// At the end of bootstrap.scss
I'll give the npm scripts a try after cleaning, however all you need to run to start developing is the
gulp command. Some of the scripts inside of package.json are related to my private build process. I'll make sure to remove them in the following update.