MD Downloads/History: unify page and toolbar background colors

chrome/browser/resources/md_downloads/toolbar.css

 /* Copyright 2015 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. */
 
 :host {
   align-items: center;
-  background: rgb(52, 73, 94);
+  background: var(--md-toolbar-color);
   color: white;
   content-sizing: padding-box;
   display: flex;
   min-height: 56px;
 }
 
 #title h1 {
   -webkit-margin-end: 0;
   -webkit-margin-start: 24px;
   font-size: 123.1%;
@@ skipping 82 matching lines @@
     display: none;
   }
 }
 
 @media all and (max-width: 1024px) {
   /* Hide #actions for narrow windows; they're shown in a vertical dot menu. */
   #actions {
     display: none;
   }
 }