Make all checkboxes uniformly darker.

chrome/browser/resources/file_manager/css/common.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. */
 
 /* This file contains "borrowed" copy of standard styles. To simplify merging,
  * when altering, please preserve original property value by adding comments.
  */
 input.common[type='checkbox'],
 input.common[type='radio'] {
   -webkit-appearance: none;
   -webkit-border-radius: 1px;
   -webkit-box-sizing: border-box;
-  border: 1px solid #c6c6c6;
+  border: 1px solid #555;
   border-radius: 1px;
   box-sizing: border-box;
   cursor: default;
   height: 13px;
   margin: 0;
+  opacity: 0.4;
   width: 13px;
 }
 
+input.common[type='checkbox']:hover,
+input.common[type='checkbox']:checked,
+input.common[type='radio']:hover,
+input.common[type='radio']:checked {
+  opacity: 1;
+}
+
 input.common[type='checkbox'] {
   position: relative;
 }
 
 input.common[type='checkbox']:checked::after {
   background-image: -webkit-image-set(
     url('../images/common/check_no_box.png') 1x,
     url('../images/common/2x/check_no_box.png') 2x);
   background-position: -3px -4px;
   background-repeat: no-repeat;
@@ skipping 215 matching lines @@
 menu.chrome-menu > :not(hr)[checked] {
   background-image: -webkit-image-set(
     url('../images/common/check_no_box.png') 1x,
     url('../images/common/2x/check_no_box.png') 2x);
   background-position: 2px center;
 }
 
 menu.chrome-menu > [checked]:before {
   display: none;
 }