refactor: update variables

Author: mrholek

scss/_variables.scss

@@ -1569,7 +1569,7 @@ $card-title-spacer-y:               $spacer * .5 !default;
 $card-border-width:                 $border-width !default;
 $card-border-color:                 rgba($black, .125) !default;
 $card-border-radius:                $border-radius !default;
-$card-box-shadow:                   null !default;
+// $card-box-shadow:                   null !default;
 $card-inner-border-radius:          subtract($card-border-radius, $card-border-width) !default;
 $card-cap-padding-y:                $card-spacer-y * .5 !default;
 $card-cap-padding-x:                $card-spacer-x !default;