Adjust @brand-primary alert in Customizer

Refs #15320

customize/index.html

@@ -125,7 +125,7 @@
 <!-- Customizer form -->
 
 <div id="defaults-change-alert" class="alert alert-warning alert-dismissible" role="alert">
-  <button type="button" class="close" data-dismiss="alert"><span aria-hidden="true">&times;</span><span class="sr-only">Close</span></button>
+  <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button>
   <strong>Heads up!</strong> The default value for <code>@brand-primary</code> has changed in the latest release to improve contrast and accessibility. Please double check your compiled CSS when upgrading.
 </div>