Fix bug where weird white block could appear at bottom of page.

This bug occured if the final code sample extended beyond the
explanation at the bottom of the page.

To fix it, I simply made the background color of body a soft
blue.

source/stylesheets/screen.css.scss

@@ -30,6 +30,7 @@ html, body {
   margin: 0;
   -webkit-font-smoothing: antialiased;
   @extend %default-font;
+  background-color: $main-bg;
 }
 
 ////////////////////////////////////////////////////////////////////////////////