Re-layout grid view and add check marks for selected items.

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 626 matching lines @@
 }
 
 .volume-warning .drive-text {
   flex: none;
   margin-right: 11px;
 }
 
 /* The cr.ui.Grid representing the detailed file list. */
 .thumbnail-grid {
   overflow-y: auto;
-  padding-bottom: 0;  /* For the preview panel. Will be overridden by JS. */
+  /* padding-bottom will be overridden by JS for preview panel. */
+  padding: 7px 7px 0 7px;
   width: 100%;
 }
 
 body[type='full-page'] .thumbnail-frame > .img-container {
   position: relative;
 }
 
 body[type='full-page'] .thumbnail-frame > .img-container,
 body[type='full-page'] .detail-name .detail-icon {
   cursor: pointer;
@@ skipping 21 matching lines @@
   right: 0;
 }
 
 .thumbnail-bottom .filename-label {
   flex: auto;
 }
 
 /* Styles specific for the grid view. */
 
 .thumbnail-grid .thumbnail-item {
-  -webkit-margin-start: 21px;
-  border: 3px solid transparent;  /* Selection will make the border visible. */
-  margin-top: 20px;
+  -webkit-margin-start: 12px;
+  background-color: rgb(245, 245, 245);
+  box-shadow: 0 1px 4px 0 rgba(0, 0, 0, 0.37);
+  height: 180px;
+  margin-top: 8px;
+  overflow: hidden;
   position: relative;
+  width: 180px;
 }
 
 .thumbnail-grid .thumbnail-frame {
-  background-color: rgb(245, 245, 245);
-  height: 120px;
-  overflow: hidden;
-  position: relative;
-  width: 160px;
+  height: 100%;
+  width: 100%;
 }
 
-.thumbnail-grid .thumbnail-item[selected] .thumbnail-frame,
-.thumbnail-grid .thumbnail-item.accepts .thumbnail-frame {
-  border-color: white;
+.thumbnail-grid .thumbnail-item[selected],
+.thumbnail-grid .thumbnail-item.accepts {
+  background-color: rgb(221, 242, 253);
+  box-shadow: 0 6px 10px 0 rgba(0, 0, 0, 0.3);
+}
+
+.thumbnail-grid .thumbnail-frame .checkmark {
+  background-image: -webkit-image-set(
+      url(../images/files/ui/thumbnail_checked.png) 1x,
+      url(../images/files/ui/2x/thumbnail_checked.png) 2x);
+  height: 34px;
+  left: 9px;
+  opacity: 0;
+  position: absolute;
+  top: 9px;
+  transition: opacity 220ms ease;
+  width: 34px;
+}
+
+.thumbnail-grid .thumbnail-item[selected] .thumbnail-frame .checkmark {
+  opacity: 1;
 }
 
 .thumbnail-grid .img-container {
   height: 100%;
   width: 100%;
 }
 
 .thumbnail-grid .thumbnail-bottom {
   background: rgba(0, 0, 0, 0.55);
   color: #fff;
@@ skipping 40 matching lines @@
 
 #default-actions-list > li > * {
   background-position: 5px center;
   background-repeat: no-repeat;
   background-size: 16px 16px;
   padding-left: 26px;
 }
 
 #list-container list > li[selected],
 #list-container list > li:active,
-#list-container grid > li[selected],
-#list-container grid > li:active,
 #default-actions-list > li[selected] {
   background-color: rgb(225, 225, 225);
 }
 
 #list-container list:focus > li[selected],
-#list-container grid:focus > li[selected],
 #default-actions-list:focus > li[selected] {
   background-color: rgb(66, 129, 244);
   color: white;
 }
 
-#list-container list > li.accepts[selected],
-#list-container grid > li.accepts[selected] {
+#list-container list > li.accepts[selected] {
   background-color: rgb(215, 215, 215);
 }
 
-#list-container list:focus > li.accepts[selected],
-#list-container grid:focus > li.accepts[selected] {
+#list-container list:focus > li.accepts[selected] {
   background-color: rgb(48, 125, 254);
 }
 
-#list-container list > li.accepts,
-#list-container grid > li.accepts {
+#list-container list > li.accepts {
   background-color: #f1f1f1;
 }
 
 #list-container.nohover grid > .accepts {
   background-color: transparent;
 }
 
 #directory-tree .tree-item.accepts > .tree-row,
 #list-container list > li.accepts,
 #list-container grid > li.accepts {
@@ skipping 977 matching lines @@
   padding: 20px 15px;
 }
 
 .text-measure {
   pointer-events: none;
   position: absolute;
   top: 0;
   visibility: hidden;
   z-index: -1;
 }