I've tried modifying and overriding the lib/web/mage/menu.js file whith my own file (which loads fine) in my custom theme with no success (using requirejs-config.js as stated in the documentation)...
Solved! Go to Solution.
First step was to find the _navigation.less file from theme-frontend-blank and copy it in my main module (located in app/design/frontend) in web/css/source. I then switched to standard media query declarations to change the standard 768px break point :