Separator is indistinct when not standalone.

@Override public void start(Stage stage) {
        VBox root = new VBox(10);
        ToolBar bar = new ToolBar();
        bar.getItems().addAll(new Button("1"), new Separator(), new Button("2"));
        root.getChildren().addAll(bar);
        Scene scene = new Scene(root, 300, 300);
        stage.setScene(scene);
        stage.show();
    }