Add a first-class off-store install UI to chrome://extensions/.

chrome/browser/resources/extensions/extensions.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.loading * {
   -webkit-transition-duration: 0 !important;
 }
 
 /* Developer mode */
 
@@ skipping 186 matching lines @@
 /* Overlays */
 
 #overlay {
   z-index: 5;
 }
 
 #overlay .page:not(.showing) {
   display: none;
 }
 
+#dropTargetOverlay {
+  color: rgb(48, 57, 66);
+  font-size: 18px;
+  text-align: center;
+}
+
+#dropTargetOverlay div {
+  margin: 1em;
+}
+
 /* Trash */
 
 #extension-settings .trash {
   -webkit-transition: opacity 200ms;
   height: 22px;
   opacity: 0.8;
   position: absolute;
   right: 0;
   top: 3px;
   vertical-align: middle;
 }
 
 html[dir='rtl'] #extension-settings .trash {
   left: 0;
   right: auto;
 }
 
 .extension-list-item:not(:hover) .trash:not(:focus) {
   opacity: 0;
 }
 
 .extension-list-item-wrapper.may-not-disable .trash {
   visibility: hidden;
 }