Remove transparency from the bottom bar.

ui/file_manager/file_manager/foreground/css/file_manager.css

 /* Copyright (c) 2014 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. */
 
 /* The order of z-index:
  * - -1: text-measure
  * - 2: drag-selection-border
  * - 3: preview-panel
  * - 500: scrollbar
  * - 500: splitter
@@ skipping 929 matching lines @@
 
 /* The cr.ui.Table representing the detailed file list. */
 .detail-table {
   width: 100%;
 }
 
 /* Bottom pane describing current selection. */
 .preview-panel {
   -webkit-transition: background-color 150ms ease;
   align-items: center;
-  background: linear-gradient(
-      to bottom, rgba(255, 255, 255, 0.8), rgba(255, 255, 255, 1));
+  background: white;
   border-top: 1px solid rgba(20, 20, 22, 0.1);
   bottom: 0;
   display: flex;
   flex-direction: row;
   height: 51px;
   left: 0;
   opacity: 1;
   padding: 0 10px 0 7px;
   position: absolute;
   right: 0;
@@ skipping 973 matching lines @@
   visibility: hidden;
 }
 
 .text-measure {
   pointer-events: none;
   position: absolute;
   top: 0;
   visibility: hidden;
   z-index: -1;
 }