ChromeOS OOBE: Update Welcome screen colors after UI review.

chrome/browser/resources/chromeos/login/oobe_welcome.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. */
 
+:root {
+  --oobe-dialog-list-item-border: 1px solid rgba(0, 0, 0, 0.06);
+}
+
 /************* Language Screen **************/
 #languageScreen .language-selection-entry {
-  border-top: 1px solid lightgrey;
+  border-top: var(--oobe-dialog-list-item-border);
   height: 44px;
   padding: 0 10px;
 }
 
 #languageScreen .language-selection-entry:last-of-type {
-  border-bottom: 1px solid lightgrey;
+  border-bottom: var(--oobe-dialog-list-item-border);
 }
 
 #languageScreen .language-selection-title {
   color: rgba(0, 0, 0, 0.87);
   font: 13px Roboto, sans-serif;
 }
 
 #languageScreen .bottom-buttons {
   padding: 0 24px;
 }
 
 /************* Accessibility Screen **************/
 #accessibilityScreen oobe-a11y-option {
-  border-top: 1px solid lightgrey;
+  border-top: var(--oobe-dialog-list-item-border);
   min-height: 56px;
 }
 
 #accessibilityScreen oobe-a11y-option:last-of-type {
-  border-bottom: 1px solid rgba(0, 0, 0, 0.14);
+  border-bottom: var(--oobe-dialog-list-item-border);
 }
 
 #accessibilityScreen .bottom-buttons {
   padding: 0 24px;
 }
 
 /************* Network Selection Screen **************/
 #networkSelectionScreen .footer {
   max-height: 313px;
 }
 
 #networkSelectionScreen .bottom-buttons {
   padding: 0 6px; /* = 8px - 2px back button border */
 }
 
 /************* Timezone Screen **************/
 #timezoneScreen .timezone-selection-entry {
-  border-top: 1px solid lightgrey;
+  border-top: var(--oobe-dialog-list-item-border);
   height: 44px;
   padding: 0 20px;
 }
 
 #timezoneScreen .timezone-selection-entry:last-of-type {
-  border-bottom: 1px solid lightgrey;
+  border-bottom: var(--oobe-dialog-list-item-border);
 }
 
 #timezoneScreen .timezone-selection-title {
   color: rgba(0, 0, 0, 0.87);
   font: 13px Roboto, sans-serif;
 }
 
 #timezoneScreen .bottom-buttons {
   padding: 0 24px;
 }