Make checkboxes in FileManager darker.

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 1029 matching lines @@
 #list-container li.thumbnail-item:hover .file-checkbox {
   opacity: 0.5;
 }
 
 .file-checkbox:hover,
 .pin:hover,
 #list-container li.table-row[selected] .file-checkbox,
 #list-container li.table-row[selected] .pin,
 #list-container li.table-row .pin:checked,
 #list-container li.thumbnail-item[selected] .file-checkbox {
-  opacity: 1 !important;
+  opacity: 0.8 !important;
 }
 
 #list-container li.table-row,
 #default-actions-list li {
   border: none;
   border-top: 1px solid transparent;
   height: 39px;
   padding-bottom: 1px;
   padding-top: 1px;
 }
@@ skipping 68 matching lines @@
 
 .detail-name > * {
   -webkit-box-align: center;
   display: -webkit-box;
 }
 
 /* Overriding input.common[type='checkbox'] rule in common.css. */
 .detail-name > input.common[type='checkbox'] {
   -webkit-margin-end: 4px;
   -webkit-margin-start: -1px;
+  border-color: #444;
 }
 
 list .detail-name > .file-checkbox::before,
 .pin::before {
   /* Invisible area that reacts on mouse events. */
   content: '';
   display: -webkit-box;
   height: 38px;
   left: -8px;
   position: absolute;
@@ skipping 367 matching lines @@
   height: 100%;
 }
 
 #list-container .table-header-cell:hover {
   background-color: #f2f2f2;
 }
 
 button:focus {
   outline-color: rgb(77, 144, 254);
 }