Code to reproduce :

@Override
    public void start(Stage stage) throws Exception {
        BorderPane root = new BorderPane();
        HBox panel = new HBox(40);
        Button b = new Button("Set style");
        final DatePicker datePicker1 = new DatePicker();
        final DatePicker datePicker2 = new DatePicker();
        panel.getChildren().addAll(datePicker1, datePicker2);
        Scene scene = new Scene(root, 500, 500);
        b.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {
                datePicker1.setStyle("-fx-shape:'M 100 100 L 150 200 L 200 150 Z';-fx-position-shape:false;-fx-background-color:blue");
            }
        });
        root.setTop(b);
        root.setCenter(panel);
        stage.setScene(scene);
        stage.show();
        stage.setTitle(VersionInfo.getRuntimeVersion());
    }