convert ::before & after back to single :

even though it's the 'wrong' way to do it, there are still too many edge cases in browsers where they prefer the single : to ::

ALSO

add line-height: 1 to nav anchors

add line-height: normal to buttons for browser consistency

add $btn-text-2 for hover/active button text colors

allow transition and transform mixins to accept more than a single value