change(listing): use a transparent background if no background is pas…

…sed to listing summary

- this way if we add a listing inside a content-box the bg color can also come from the content box

src/less/listing-cards.less

@@ -142,7 +142,7 @@ each(range(5), {
 }
 
 .listing.summary {
-  background-color: var(--bg-color, #fff);
+  background-color: var(--bg-color, transparent);
   color: var(--text-color, @tertiaryColor);
 }