Revert $card-color to default (null)

Not overriding $card-color fixes the toggling between light and dark. This reverts to Bootstrap default behaviors.

Author: Carl-Hugo

scss/_dark-variables.scss

@@ -57,7 +57,7 @@ $border-color: $dark;
 // Cards
 $card-border-color: rgba($white, .125);
 $card-cap-bg: rgba($white, .03);
-$card-color: $gray-200;
+// $card-color: $gray-200;
 $card-bg: $gray-900; //$body-bg ?
 
 $nav-tabs-border-color: rgba($white, .125);