public void start(Stage stage) throws Exception {

    Parent root = FXMLLoader.load(getClass().getResource("Preview.fxml"));
    Scene scene = new Scene(root);
    stage.setScene(scene);
    stage.setTitle("Ejemplo Drag And Drop");
    stage.show();
}

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    launch(args);
}

.mainFxmlClass {
#pane{
    -fx-background-size: 800 900;
    -fx-background-radius: 0 0 18 18;
    -fx-border-radius: 0 0 18 18;
    -fx-border-width:5;
    -fx-background-color: #FC3D44;
}