Plugins: Add visible focus on plugin modal close button.

Change the focus and hover state of the plugin modal close button to use `#f0f0f1` with visible outline to be visible against modal overlay.

Props dperonne, viralsampat, jeawhanlee, mukesh27, sabernhardt, joedolson.
Fixes #56604.

git-svn-id: https://develop.svn.wordpress.org/trunk@55107 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson

src/wp-admin/css/common.css

@@ -3030,11 +3030,17 @@ div.action-links {
 
 .plugin-details-modal #TB_closeWindowButton:hover,
 .plugin-details-modal #TB_closeWindowButton:focus {
-	color: #135e96;
 	outline: none;
 	box-shadow: none;
 }
 
+.plugin-details-modal #TB_closeWindowButton:hover::after,
+.plugin-details-modal #TB_closeWindowButton:focus::after {
+	outline: 2px solid;
+	outline-offset: -4px;
+	border-radius: 4px;
+}
+
 .plugin-details-modal .tb-close-icon {
 	display: none;
 }