Accessibility: Improve the color contrast ratio of the dismiss notice buttons.

Props chris_dev.
Fixes #39970.

Built from https://develop.svn.wordpress.org/trunk@40570


git-svn-id: http://core.svn.wordpress.org/trunk@40440 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/common-rtl.css

@@ -779,7 +779,7 @@ img.emoji {
 #bulk-titles div a:before,
 .notice-dismiss:before {
 	background: none;
-	color: #b4b9be;
+	color: #72777c;
 	content: "\f153";
 	display: block;
 	font: normal 16px/20px dashicons;
@@ -1417,7 +1417,7 @@ div.error p,
 	margin: 0;
 	padding: 9px;
 	background: none;
-	color: #b4b9be;
+	color: #72777c;
 	cursor: pointer;
 }