public class RichTextFxTest extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        var textArea1 = new InlineCssTextArea();
        prepare(textArea1);
        var textArea2 = new InlineCssTextArea();
        prepare(textArea2);

        textArea1.getStylesheets().clear();//LINE X

        var box = new VBox(new Label("TextArea1"), textArea1, new Label("TextArea2"), textArea2);
        var scene = new Scene(box, 300, 300);
        stage.setScene(scene);
        stage.show();
    }

    private void prepare(InlineCssTextArea textArea) {
        textArea.appendText("Default style string\n");
        textArea.append("Current style string\n",
                "-rtfx-underline-color: black;-rtfx-underline-width: 1px;-rtfx-underline-offset: 2px");
    }
}