done : )

src/main/java/rocks/zipcode/calcskin/CalcSkin.java

@@ -60,7 +60,7 @@ public class CalcSkin extends Application {
     private VBox createLayout(TextField screen, TilePane buttons) {
         final VBox layout = new VBox(20);
         layout.setAlignment(Pos.CENTER);
-        layout.setStyle("-fx-background-color: silver; -fx-padding: 20; -fx-font-size: 20;");
+        layout.setStyle("-fx-background-color: #e6f94f; -fx-padding: 20; -fx-font-size: 20;");
         layout.getChildren().setAll(screen, buttons);
         handleAccelerators(layout);
         screen.prefWidthProperty().bind(buttons.widthProperty());