Update --selection bg-color when default browser theme is dark

The --selection bg-color when default theme for browser is dark, and the user hasn't changed the theme at all (thus theme-icon-when-auto shows up) is $(green-medium) but the contrast is way too terrible (the actual value is 2.35 which is way too low)

Author: ZougaRida

djangoproject/scss/_dark-mode.scss

@@ -107,7 +107,7 @@ html[data-theme="light"],
         --search-link-hover: #{$green-light};
         --search-mark-text: #{$black-light-5};
 
-        --selection: #{$green-medium};
+        --selection: #{$green-dark};
 
         --community-img-bg: #{$green-dark};
         --community-img-fg: #{$white};