Replace IDR_WEB_UI_CLOSE* with IDR_CLOSE_DIALOG*, cleanup, etc.

chrome/browser/resources/shared/css/dialogs.css

 /* Copyright (c) 2012 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. */
 
 .cr-dialog-container {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   -webkit-transition: opacity 250ms linear;
   -webkit-user-select: none;
   display: -webkit-box;
@@ skipping 109 matching lines @@
 }
 
 .cr-dialog-buttons {
   -webkit-box-orient: horizontal;
   -webkit-box-pack: end;
   display: -webkit-box;
   padding-top: 10px;
 }
 
 .cr-dialog-close {
-  background: url('chrome://theme/IDR_WEB_UI_CLOSE') center no-repeat;
+  background: url('chrome://theme/IDR_CLOSE_DIALOG') center no-repeat;
   display: inline-block;
   height: 44px;
   opacity: 0.7;
   position: absolute;
   right: 0;
   top: 0;
   width: 44px;
 }
 
 .cr-dialog-close:hover {
-  background-image: url('chrome://theme/IDR_WEB_UI_CLOSE_HOVER');
+  background-image: url('chrome://theme/IDR_CLOSE_DIALOG_H');
 }
 
 .cr-dialog-close:active {
-  background-image: url('chrome://theme/IDR_WEB_UI_CLOSE_PRESSED');
+  background-image: url('chrome://theme/IDR_CLOSE_DIALOG_P');
 }