@dark Styles for Border Colors

[badrobotbrain]

Add @dark styles for border color declarations so designers can define an alternate color for borders when users are in dark mode.

For example: a scarlet border agains a cream background looks good, but doesn't have as much visual impact when the user is in dark mode and the scarlet border is against a dark gray background. As the designer, I would like to define what color that border changes to in dark mode myself.