ThemeManager#setCurrentThemeId(String)

@Component(service=Theme.class)
public class DefaultTheme extends AbstractTheme {
	public DefaultTheme() {
		super("theme.default", "Default theme", DefaultTheme.class.getClassLoader().getResource("css/default.css"));
		
		System.out.println("Default Theme loaded.");
	}
	
	@Reference(cardinality=ReferenceCardinality.MULTIPLE)
	@Override
	public void registerStylesheet(Stylesheet stylesheet) {
		System.out.println("Default Registration.");
		super.registerStylesheet(stylesheet);
	}

	@Override
	public void unregisterStylesheet(Stylesheet stylesheet) {
		System.out.println("Default Deregistration.");
		super.unregisterStylesheet(stylesheet);
	}
}

public static void switchTheme(String themeID) {
	themeManager.setCurrentThemeId(themeID);
}
	
public static Theme getTheme() {
	return themeManager.getCurrentTheme();
}

public class DefaultTheme extends AbstractTheme {
	
	public DefaultTheme() {
		
	super("theme.default", "Default theme", DefaultTheme.class.getClassLoader().getResource("css/default.css"));
		
}

@PostConstruct
public void init(BorderPane root) {
       try {			
		Button btn = new Button("Switch Theme");
		root.setCenter(btn);
		btn.addEventHandler(MouseEvent.MOUSE_CLICKED, new EventHandler<Event>() {

			@Override
			public void handle(Event event) {
				System.out.println("Set night theme.");

				LifeCycleManager.switchTheme("theme.night");
			}
		});			
			
	}catch (Exception e) {
		e.printStackTrace();

@PostConstruct
public void init(Stage primStage) {
	try {	
			
		BorderPane root= new BorderPane();
		Button btn = new Button("Switch Theme");
		root.setCenter(btn);
		btn.addEventHandler(MouseEvent.MOUSE_CLICKED, new EventHandler<Event>() {

			@Override
			public void handle(Event event) {
				System.out.println("Set night theme.");

				LifeCycleManager.switchTheme("theme.night");
			}
		});	
			
		primStage.setScene(new Scene(root));
			
	}catch (Exception e) {
		e.printStackTrace();
	}