Add theme variable to specify color-scheme for :root (#1280)

Previously, the color scheme information was not passed on to the
browser. This could result in scrollbars being light, when the dark
theme is in use.

Now, `:root { color-scheme: $color-scheme; }` is specified.
This will ensure the color theme is enforced.

Ref: https://developer.mozilla.org/en-US/docs/Web/CSS/color-scheme

_sass/color_schemes/dark.scss

@@ -1,3 +1,4 @@
+$color-scheme: dark;
 $body-background-color: $grey-dk-300;
 $body-heading-color: $grey-lt-000;
 $body-text-color: $grey-lt-300;

_sass/color_schemes/light.scss

@@ -1,3 +1,4 @@
+$color-scheme: light !default;
 $body-background-color: $white !default;
 $body-heading-color: $grey-dk-300 !default;
 $body-text-color: $grey-dk-100 !default;