ChromeOS OOBE: Fix OK button on Welcome screen.

chrome/browser/resources/chromeos/login/oobe_welcome_secondary_button.css

 /* Copyright 2016 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. */
 :host {
   color: #5a5a5a;
   height: 32px; /* = 28 + 2 * 2px keyboad-focus border */
   max-width: 300px;
   min-width: 120px;
 }
 
 #button {
   --paper-button-flat-keyboard-focus: {
-    border-color: rgba(66, 133, 244, 0.4); /* #4284f4 */
+    border-color: var(--oobe-text-button-focused-border-color);
     border-radius: 2px;
     border-style: solid;
     border-width: 2px;
   };
   height: 100%;
   margin: 0;
   padding: 0;
   width: 100%;
 }
 
@@ skipping 20 matching lines @@
   font-size: 12px;
   text-transform: none;
 }
 
 iron-icon {
   --iron-icon-height: 16px;
   --iron-icon-width: 16px;
   color: #5a5a5a;
   margin: 0 8px 0 0;
 }