Customizable inverse bg color

Created by: henwen

Currently, we can give a component an inverse background color using the .bg-inverse class. However, .bg-inverse is hard set to use background-color: $gray-dark.

Non-gray dark bg's are pretty common, so It'd be great to have a $background-color-inverse sass var in _variables.scss that could be referenced directly, and by utility classes like bg-inverse.