The class ".border" (css/utilities/_borders.scss) is set the border-color to "$gray-200".
So I see that it would be nice to create a variable for the border color "$border-color". Because the default border should not always be "gray-200".
Another thing, there is a "$border-width" variable. Would not it be better to use this variable to set the default width of the border?
* Clean up some utilities
- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies
* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication
* Fix outdated classes to use newer ones
- Move and rename .img-rounded to .rounded, .img-circle to .rounded-circle
- Add new .rounded-{direction} utils
- New docs pages for border utils with TBD comments for the border property
- Removes most image examples for rounding from the content/images docs in favor of new docs page
