dark_theme: Declare color-scheme CSS property.

This allows the browser to select a more appropriate color for the
browser scrollbars, for example.

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

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/styles/dark_theme.css

@@ -6,6 +6,8 @@
      */
     @extend %dark-theme-block;
 
+    color-scheme: dark;
+
     body {
         color: hsl(236, 33%, 90%);
     }

static/styles/zulip.css

@@ -49,6 +49,10 @@ body,
     transition: background-color 200ms linear;
 }
 
+:root {
+    color-scheme: light;
+}
+
 input,
 button,
 select,