Add border to promo banners in high contrast themes

Fixes #1596 (to the extent that we can)

public/css/product-promos.css

@@ -8,6 +8,7 @@
   background-position: 104%;
   background-size: auto 104%;
   background-repeat: no-repeat;
+  border: 1px solid transparent;
 }
 
 .promo-copy {