Add default to $main-footer-bg (#2394)

build/scss/_variables.scss

@@ -97,7 +97,7 @@ $main-footer-height-inner: (($font-size-root * $line-height-base) + ($main-foote
 $main-footer-height: calc(#{$main-footer-height-inner} + #{$main-footer-border-top-width}) !default;
 $main-footer-height-sm-inner: (($font-size-sm * $line-height-base) + ($main-footer-padding-sm * 2)) !default;
 $main-footer-height-sm: calc(#{$main-footer-height-sm-inner} + #{$main-footer-border-top-width}) !default;
-$main-footer-bg: $white;
+$main-footer-bg: $white !default;
 
 // SIDEBAR SKINS
 // --------------------------------------------------------