Set :root color-scheme dark or light

[Banner-Keith]

I have a minor request to add the color-scheme value to :root. This allows the browser to know to make the scroll bar dark and other related effects.

So on all dark themes you'd just have to add:

:root { color-scheme: dark; }

and for light themes you'd add

:root { color-scheme: light; }

I would be happy to open a PR with this change if you're okay with it.