[webui-ntp] Show background for focused menu item

Previously, we showed an outline. Changed per UX request.

Bug: 1040256
Change-Id: I23115e42361634c42bcb7a43e81638af2b88c6db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995946
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731580}

chrome/browser/resources/new_tab_page/customize_dialog.html

@@ -19,7 +19,6 @@
   .menu-item {
     align-items: center;
     border-radius: 0 16px 16px 0;
-    box-sizing: border-box;
     cursor: pointer;
     display: flex;
     flex-direction: row;
@@ -44,13 +43,7 @@
   }
 
   :host-context(.focus-outline-visible) .menu-item:focus {
-    border-color: rgba(var(--google-blue-600-rgb), .4);
-    border-style: solid solid solid none;
-    border-width: 2px;
-  }
-
-  :host-context(.focus-outline-visible[dir=rtl]) .menu-item:focus {
-    border-style: solid none solid solid;
+    background-color: rgba(var(--google-grey-900-rgb), .16);
   }
 
   .menu-item-icon {