Admin: Update generic focus styles to use CSS variables.

Replace hardcoded color values in the common link focus styles
with CSS custom properties to match the pattern used in buttons
and form elements.

Author: fabiankaegy

src/wp-admin/css/common.css

@@ -279,8 +279,8 @@ a:focus,
 a:focus .media-icon img,
 a:focus .plugin-icon,
 .wp-person a:focus .gravatar {
-	color: #043959;
-	box-shadow: 0 0 0 2px #2271b1;
+	color: var(--wp-admin-theme-color-darker-20, #183ad6);
+	box-shadow: 0 0 0 var(--wp-admin-border-width-focus, 1.5px) var(--wp-admin-theme-color, #3858e9);
 	/* Only visible in Windows High Contrast mode */
 	outline: 2px solid transparent;
 }