Request: Dark mode as opt-in

We don't have an requirements to create a "Dark mode" variant of our website. However when dark mode is preferred certain aspects of the base styles do update due to `@media (--OSdark) {`.

The prop is fine but could the rules be refactored into an optional "extra" that gives us an opt-in approach, similar to normalise and buttons etc.? Thank you.

Locate the base styles containing `@media (--OSdark)` and compare their organization with the optional extras mentioned in the issue, such as normalize and buttons. Done means dark-mode rules are separated into an opt-in extra rather than being applied automatically.

Written by the indexing model from the issue text.