Fix edge case issue where tutorials-overview bg may appear light. (#719)

In situations with no footer and a long viewport height and little
actual content, the background of the empty space will be white.

It should be a darker fill color in any light/dark/auto appearance since
that page purposefully has an "always-dark" design theme.

Resolves: rdar://108634043

Author: mportiz08

src/components/TutorialsOverview.vue

@@ -121,6 +121,8 @@ export default {
 @import 'docc-render/styles/_core.scss';
 
 .tutorials-overview {
+  background: dark-color(fill);
+  flex: 1;
   height: 100%;
 
   .radial-gradient {