implemented basic functions, changed appearance

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

     }
 
     public double add(double v, double v1) {
-        return Double.NaN;
+        return v+v1;
     }
 
     public double subtract(double v, double v1) {
-        return Double.NaN;
+        return v-v1;
     }
 
     public double multiply(double v, double v1) {
-        return Double.NaN;
+        return v*v1;
     }
 
     public double divide(double v, double v1) {
-        return Double.NaN;
+        return v/v1;
     }
 }

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

     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: gold; -fx-padding: 20; -fx-font-size: 20;");
         layout.getChildren().setAll(screen, buttons);
         handleAccelerators(layout);
         screen.prefWidthProperty().bind(buttons.widthProperty());
 
     private TextField createScreen() {
         final TextField screen = new TextField();
-        screen.setStyle("-fx-background-color: aquamarine;");
+        screen.setStyle("-fx-background-color: green;");
         screen.setAlignment(Pos.CENTER_RIGHT);
         screen.setEditable(false);
         screen.textProperty().bind(Bindings.format("%.0f", currentValue));