The following app produces the following warning, when using the Menu:

(tested with 8u20 b13)

WARNING: Caught 'java.lang.ClassCastException: javafx.scene.paint.LinearGradient cannot be cast to javafx.scene.paint.Color' while converting value for '-fx-text-fill' from rule '*.menu-bar>*.container>*.menu-button:hover>*.label' in stylesheet jar:file:/C:/Program%20Files%20(x86)/Java/jdk1.8.0_20/jre/lib/ext/jfxrt.jar!/com/sun/javafx/scene/control/skin/modena/modena.bss


Nonetheless, the blue-red gradient works as expected.


import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.stage.Stage;

public class TestApp2 extends Application {
    public static void main(String[] args) {
        Application.launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {

        MenuBar menuBar = new MenuBar();
        Menu menu1 = new Menu("Menu");
        menu1.getItems().add(new MenuItem("Test"));

        menuBar.getMenus().add(menu1);

        Scene scene = new Scene(menuBar);
        scene.getStylesheets().add("styles.css");
        stage.setScene(scene);
        stage.show();
    }
}

styles.css:

.root {
    -fx-selection-bar: linear-gradient(to top, derive(red, -2%) 0%, derive(blue, 50%) 100%);
}