Read the whole article by Mike Herchel

To get the breakpoint value as a number instead of a text string:
use .valuesAsNumber in the breakpoint.refreshValue function, as well as in the if statements. Remember to also change the content:'device' lines to contain the numbers (for example content:'700')in scss.

Having the breakpoints as numbers means it'll be easier to use comparing operators for when something needs to encompass multiple breakpoints.