Replace hard coded value of body-background-color with white variable (#1185)

sass/base/generic.sass

@@ -1,4 +1,4 @@
-$body-background-color: #fff !default
+$body-background-color: $white !default
 $body-size: 16px !default
 $body-rendering: optimizeLegibility !default
 $body-family: $family-primary !default