From Meteor 1.3.2 you can import .scss files from node_modules (#6037) so finally we can use Foundation 6 and Bootstrap 4 from Npm packages!
Here are my 2 test repos and it seems to work well.
(You’ll find some guides in the README.md files.)
Foundation 6: https://github.com/juliancwirko/meteor-npm-foundation-test
Bootstrap 4: https://github.com/juliancwirko/meteor-bootstrap-npm-test