Add a box-shadow to the app-bar per material design

Also made the background a MD color.

R=abarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/951653004

examples/stocks/stock.sky

@@ -28,6 +28,7 @@
   #change {
     border-radius: 5px;
     min-width: 72px;
+    padding: 2px;
     padding-right: 10px;
     text-align: right;
   }

examples/stocks/stocks.sky

@@ -30,9 +30,10 @@
     sky-toolbar {
       display: flex;
       align-items: center;
-      background-color: #526E9C;
+      background-color: #3F51B5;
       color: white;
       height: 56px;
+      box-shadow: 0px 4px 4px grey;
     }
     #menu {
       display: flex;