Files.app: Add gradient to the background of the preview panel.

chrome/browser/resources/file_manager/css/file_manager.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. */
 
 html {
   height: 100%;
 }
 
 html.col-resize * {
   cursor: col-resize !important;
@@ skipping 1452 matching lines @@
   opacity: 1;
   padding: 0 16px 0 7px;
   position: relative;
   z-index: 3;
 }
 
 body[new-ui] .preview-panel {
   -webkit-box-align: center;
   -webkit-box-orient: horizontal;
   -webkit-transition: background-color 150ms ease;
-  background: rgba(255, 255, 255, 0.9);
+  background: linear-gradient(
+      to bottom, rgba(255, 255, 255, 0.8), rgba(255, 255, 255, 1));
   border-top: 1px solid rgb(225, 225, 225);
   bottom: 0;
   display: -webkit-box;
   height: 51px;
   left: 0;
   opacity: 1;
   padding: 0 12px 0 7px;
   position: absolute;
   right: 0;
   z-index: 3;
@@ skipping 976 matching lines @@
 body[new-ui] #iframe-drag-area {
   -webkit-app-region: drag;
   height: 45px;
   left: 64px;
   position: absolute;
   right: 70px;
   top: 0;
   width: auto;
   z-index: 101;
 }