public class Main extends Application {

    public static void main(String[] args) {
        Application.setUserAgentStylesheet(STYLESHEET_MODENA);
        Application.launch(Main.class, args);
    }
...

If you move the call to setUserAgentStylesheet to the start(Stage) method, everything is fine.

Using Modena Theme
Exception in thread "main" java.lang.NullPointerException
at com.sun.javafx.tk.quantum.QuantumToolkit.isSupported(QuantumToolkit.java:1208)
at com.sun.javafx.application.PlatformImpl.isSupported(PlatformImpl.java:447)
at com.sun.javafx.application.PlatformImpl$11.run(PlatformImpl.java:543)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.setPlatformUserAgentStylesheet(PlatformImpl.java:539)
at javafx.application.Application.setUserAgentStylesheet(Application.java:434)
at fxtest.Main.main(Main.java:40)