Admin: Match welcome panel to other dashboard widgets.

Set the Welcome Panel to have the same border color as the other dashboard widgets following the admin reskin.

Props marc4, fabiankaegy, joedolson.
Fixes #64741.

git-svn-id: https://develop.svn.wordpress.org/trunk@61758 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson

src/wp-admin/css/dashboard.css

@@ -140,7 +140,7 @@
 	position: relative;
 	overflow: auto;
 	margin: 16px 0;
-	background-color: #151515;
+	background-color: #c3c4c7;
 	border: 1px solid rgb(0, 0, 0, 0.1);
 	border-radius: 8px;
 	font-size: 14px;