Specify color-scheme for each theme

[Dubzer]

This PR adds a CSS property color-scheme to _schemes.scss.
It allows the browser to apply the appropriate default user-agent styles for the current theme. For example, to use dark scrollbar on Windows in chromium-based browsers.

Learn more about this property: https://developer.mozilla.org/en-US/docs/Web/CSS/color-scheme

Here's an example of improvement in scrollbar style for dark themes on Windows in MS Edge:

Before	After

[syphar]

r? @Nemo157 @GuillaumeGomez

[Nemo157]

Interesting, this doesn't seem to do much in firefox because rustdoc sets the scrollbar-color overriding it (to something that's hard to see with just the thumb visible in dark 😔), and even if it didn't firefox appears to base the scrollbar color on the background-color, but chromium browsers and safari don't support setting it yet and I guess don't auto-derive a good color so this is useful in them (and probably for any other effects it might have).

[Nemo157]

I'd still like @GuillaumeGomez to take a look too.

[GuillaumeGomez]

I'll try to take a look by the end of the week.

[GuillaumeGomez]

Looks all good to me, thanks!