Increase contrast in dark-mode (twbs#38525)

This increases the default brightness for the body text-color in dark
mode, which inceases readability a bit.

It was also suggested to darken the background a bit, but we
unfortunately don't have a darker gray in our color palette. So for now,
let's increase the text-color by two notches, and leave the background
color as-is instead.

scss/_variables-dark.scss

@@ -40,7 +40,7 @@ $light-border-subtle-dark:          $gray-700 !default;
 $dark-border-subtle-dark:           $gray-800 !default;
 // scss-docs-end theme-border-subtle-dark-variables
 
-$body-color-dark:                   $gray-500 !default;
+$body-color-dark:                   $gray-300 !default;
 $body-bg-dark:                      $gray-900 !default;
 $body-secondary-color-dark:         rgba($body-color-dark, .75) !default;
 $body-secondary-bg-dark:            $gray-800 !default;