[Media Router WebUI] Fix remaining AX_COLOR_01 warnings.

chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 .blocking {
   background-color: white;
   overflow: hidden;
   position: relative;
   text-align: center;
 }
@@ skipping 23 matching lines @@
   padding: 16px;
 }
 
 .non-blocking > #buttons {
   display: flex;
   flex-direction: row;
   justify-content: flex-end;
   width: 100%;
 }
 
+.non-blocking > #buttons > .button {
+  color: var(--paper-blue-300);
+}
+
+.non-blocking > #buttons > #default-button {
+  -webkit-margin-end: 24px;
+}
+
 .non-blocking > div > #title {
   color: rgba(255, 255, 255, 0.87);
 }
 
 paper-button {
   margin: 0;
 }