Optimize simulator for light mode. (#379)

* Optimize simulator for light mode.

* Add user-select none.

sim/public/simulator.html

@@ -18,6 +18,10 @@ body {
   overflow: hidden;
   margin: 0;
 }
+
+.light * {
+  transition: none !important;
+}
     </style>
     <script src="/cdn/bluebird.min.js"></script>
     <script src="/cdn/pxtsim.js"></script>