warn about change default value in customizer

customize/index.html

@@ -109,6 +109,7 @@
 -->
 <!--[if lt IE 9]>
   <style>
+    #defaults-change-alert,
     .bs-customizer,
     .bs-customizer-import,
     .bs-docs-sidebar {
@@ -123,6 +124,11 @@
 
 <!-- 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>
+  <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>
+
 <div class="bs-docs-section bs-customizer-import">
   <div id="import-drop-target" class="bs-dropzone">
     <h2>✍</h2>