Boxes are one of our earliest components in our CSS library. We've had

it since early in the 1.0 timeframe. They were used more for

presentation, and over time, we've reduced usage of the actual classes

and moved to repeated definitions within other CSS files.
This change makes this a bit easier. We now have a namespaced set of

variables that can be used instead of the older global ones (which are

now marked deprecated, but still widely used).
There's also a few functions for adding box styles to an element.

#rb-ns-ui.boxes.make-box() will add the standard rules.

.make-box-at-screen-gte() will add them only at certain screen sizes

(useful for desktop-only views). .unmake-box() will zero out the

styles used for a box, which is going to be useful for some admin UI

work (which isn't namespaced in super consistent and useful ways).
These will be used in upcoming admin UI changes.

Used these new mixins and variable in upcoming changes.