Admin UI: Reskin notices to align with WordPress Design System

Update all notice styles to match Gutenberg's component patterns and
the WordPress Design System specifications.

**Base Notice Styles:**
- Border: 4px left border only (removed generic border)
- Box shadow: removed for cleaner appearance
- Padding: 8px 12px (was 1px 12px)
- Typography: 13px/20px, color #1e1e1e (gray-900)

**Notice Type Colors:**
- Info: #3858e9 border, transparent background
- Success: #4ab866 border, #eff9f1 background
- Warning: #f0b849 border, #fef8ee background
- Error: #cc1818 border, #fcf0f0 background

**Dismiss Button:**
- Size: 24px × 24px (was 20px × 20px)
- Position: top 8px, right 12px (improved alignment)
- Color: #1e1e1e (was #787c82)
- Icon size: 20px/24px (was 16px/20px)
- Focus: Gutenberg-style ring using var(--wp-admin-theme-color)
- Hover: opacity 0.7

**Links in Notices:**
- Color: var(--wp-admin-theme-color)
- Hover: var(--wp-admin-theme-color-darker-10)
- Focus: proper focus ring with var(--wp-admin-border-width-focus)

**Spacing:**
- Dismissible notices: 48px right padding (was 38px)
- Provides 24px gap between content and dismiss button

See: https://core.trac.wordpress.org/ticket/64308

Author: fabiankaegy

src/wp-admin/css/common.css

@@ -805,14 +805,14 @@ img.emoji {
 #bulk-titles .ntdelbutton:before,
 .notice-dismiss:before {
 	background: none;
-	color: #787c82;
+	color: #1e1e1e;
 	content: "\f153";
 	content: "\f153" / '';
 	display: block;
-	font: normal 16px/20px dashicons;
-	height: 20px;
+	font: normal 20px/24px dashicons;
+	height: 24px;
 	text-align: center;
-	width: 20px;
+	width: 24px;
 	-webkit-font-smoothing: antialiased;
 	-moz-osx-font-smoothing: grayscale;
 }
@@ -1420,11 +1420,11 @@ th.action-links {
 div.updated,
 div.error {
 	background: #fff;
-	border: 1px solid #c3c4c7;
-	border-left-width: 4px;
-	box-shadow: 0 1px 1px rgba(0, 0, 0, 0.04);
+	border: none;
+	border-left: 4px solid #c3c4c7;
+	box-shadow: none;
 	margin: 5px 15px 2px;
-	padding: 1px 12px;
+	padding: 8px 12px;
 }
 
 div[class="update-message"] { /* back-compat for pre-4.6 */
@@ -1437,15 +1437,31 @@ div.updated p,
 div.error p,
 .form-table td .notice p {
 	margin: 0.5em 0;
-	padding: 2px;
+	padding: 0;
+	font-size: 13px;
+	line-height: 20px;
+	color: #1e1e1e;
 }
 
-.error a {
+.notice a,
+.error a,
+.updated a {
+	color: var(--wp-admin-theme-color);
 	text-decoration: underline;
 }
 
-.updated a {
-	padding-bottom: 2px;
+.notice a:hover,
+.error a:hover,
+.updated a:hover {
+	color: var(--wp-admin-theme-color-darker-10);
+}
+
+.notice a:focus,
+.error a:focus,
+.updated a:focus {
+	box-shadow: 0 0 0 var(--wp-admin-border-width-focus) var(--wp-admin-theme-color);
+	outline: 2px solid transparent;
+	border-radius: 2px;
 }
 
 .notice-alt {
@@ -1463,66 +1479,82 @@ div.error p,
 }
 
 .wp-core-ui .notice.is-dismissible {
-	padding-right: 38px;
+	padding-right: 48px;
 	position: relative;
 }
 
 .notice-dismiss {
 	position: absolute;
-	top: 0;
-	right: 1px;
+	top: 8px;
+	right: 12px;
 	border: none;
 	margin: 0;
-	padding: 9px;
+	padding: 0;
 	background: none;
-	color: #787c82;
+	color: #1e1e1e;
 	cursor: pointer;
+	width: 24px;
+	height: 24px;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	border-radius: 2px;
 }
 
 .notice-dismiss:hover:before,
-.notice-dismiss:active:before,
-.notice-dismiss:focus:before {
-	color: #d63638;
+.notice-dismiss:active:before {
+	color: #1e1e1e;
+	opacity: 0.7;
 }
 
 .notice-dismiss:focus {
-	box-shadow: 0 0 0 2px #2271b1;
+	box-shadow: 0 0 0 var(--wp-admin-border-width-focus) var(--wp-admin-theme-color);
 	/* Only visible in Windows High Contrast mode */
 	outline: 2px solid transparent;
 }
 
+.notice-dismiss:focus:before {
+	color: #1e1e1e;
+}
+
 .notice-success,
 div.updated {
-	border-left-color: #00a32a;
+	border-left-color: #4ab866;
+	background-color: #eff9f1;
 }
 
-.notice-success.notice-alt {
-	background-color: #edfaef;
+.notice-success.notice-alt,
+div.updated.notice-alt {
+	background-color: #eff9f1;
 }
 
 .notice-warning {
-	border-left-color: #dba617;
+	border-left-color: #f0b849;
+	background-color: #fef8ee;
 }
 
 .notice-warning.notice-alt {
-	background-color: #fcf9e8;
+	background-color: #fef8ee;
 }
 
 .notice-error,
 div.error {
-	border-left-color: #d63638;
+	border-left-color: #cc1818;
+	background-color: #fcf0f0;
 }
 
-.notice-error.notice-alt {
-	background-color: #fcf0f1;
+.notice-error.notice-alt,
+div.error.notice-alt {
+	background-color: #fcf0f0;
 }
 
 .notice-info {
-	border-left-color: #72aee6;
+	border-left-color: #3858e9;
+	background-color: transparent;
 }
 
 .notice-info.notice-alt {
-	background-color: #f0f6fc;
+	background-color: transparent;
 }
 
 #plugin-information-footer .update-now:not(.button-disabled):before {