Change fn-stocks to use a different color toolbar.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/973133002

examples/fn/widgets/toolbar.dart

@@ -9,7 +9,7 @@ class Toolbar extends Component {
     align-items: center;
     height: 84px;
     z-index: 1;
-    background-color: #3F51B5;
+    background-color: #9C27B0; // Purple 500
     color: white;
     box-shadow: 0 3px 6px rgba(0,0,0,0.16), 0 3px 6px rgba(0,0,0,0.23);'''
   );