Add $body-bg for just in case we use it

Author: shravanngoswamii

theming/variables/_colors-light.scss

@@ -19,6 +19,7 @@ $text-color: $gray-700;
 $text-muted: $gray-600;
 
 $btn-border-color: $gray-200;
+$body-bg: $background-body;
 $navbar-bg: $white;
 $navbar-color: $text-muted;