Extract colors from wallpaper and dynamically update login screen overlay

ui/login/account_picker/md_screen_account_picker.css

 /* Copyright 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.
  */
 
 .account-picker.flying-pods #account-picker {
   transition: width 180ms ease, height 180ms ease;
 }
 
 #bubble {
@@ skipping 56 matching lines @@
   opacity: 0.8;
   transition: opacity 1s;
   visibility: visible;
 }
 
 .small-pod-container {
   overflow: scroll;
   position: absolute;
 }
 
+.small-pod-container.scroll {
+  background-color: rgba(0, 0, 0, 0.3);
+}
+
+.small-pod-container.images-loading {
+  visibility: hidden;
+}
+
 .small-pod-container::-webkit-scrollbar {
    width: 0;
 }
 
-.small-pod-container.scroll {
-  background-color: rgba(20, 29, 40, .2);
-}
-
 .small-pod-container.scroll::-webkit-scrollbar {
    width: 9px;
 }
 
 .small-pod-container.scroll::-webkit-scrollbar-thumb {
    background-color: rgba(255, 255, 255, .17);
    border-radius: 8px;
 }
 
 .small-pod-container-mask {
-  background: linear-gradient(#4FC3F7, transparent);
+  background: linear-gradient(#000, transparent);
   height: 112px;
   position: absolute;
   width: 100%;
   z-index: 2;
 }
 
+.small-pod-container-mask.images-loading {
+  visibility: hidden;
+}
+
 .small-pod-container-mask.rotate {
   transform: rotate(180deg);
 }