Change $border to $grey-lighter in mixins. Fixes #1862

[Log1x]

This is a bugfix for #1862.

Proposed solution

Change $border to $grey-lighter to allow the ability to @import mixins.sass without having to import derived-variables.sass

Tradeoffs

N/A

Testing Done

Built my project using this fork with a scenario similar to #1862 with a standalone @import of mixins.sass and am now able to build successfully.