Found it!
I searched for "breakpoint-medium" in the tm-beans/lib directory. Got a bunch of hits, but way at the bottom (in variables.less) found the definitions.
Thanks!
Hi Wyn,
Remember that this is all UIkit related, Beans only comes into play as it allows you to modify all UIkit LESS variables in your child theme (official Beans starter child theme) style.less and process it for you which makes it super easy to customize UIkit according to your projects.
Happy coding,