Index: wp-content/themes/twentyfifteen/style.css
===================================================================
--- wp-content/themes/twentyfifteen/style.css	(revision 29724)
+++ wp-content/themes/twentyfifteen/style.css	(working copy)
@@ -956,7 +956,7 @@
 }
 
 .post-navigation .has-post-thumbnail a:before {
-	background-color: rgba(0, 0, 0, 0.3);
+	background-color: rgba(0, 0, 0, 0.4);
 	content: "";
 	display: block;
 	height: 100%;