Support for prefers-color-scheme media query

Created by: rolandasb

Apple introduced prefers-color-scheme media query in Safari Technology Preview 68, which allows developers to apply different rules if user is using dark mode on Mac Mojave:

@media (prefers-color-scheme: light) { ... }
@media (prefers-color-scheme: dark) { ... }

https://trac.webkit.org/changeset/237156/webkit

Maybe it's worth supporting this in Bootstrap?