JFXSlider thumb disappear when running

[AGPEIM]

i put a jfxSlider in a hbox and it can display well in Scene Builder,however ,the thumb doesn't show when it's running.
i'm using java11 and javafx 15.0.1 ,here is the fxml code
<HBox layoutX="20"> <JFXSlider orientation="VERTICAL"/> </HBox>

sorry for my poor English

[AGPEIM]

I loaded the css flies in a wrong way ,it should be like scene.getStylesheets().add(getClass().getResource("Style/style.css").toExternalForm()); but i use Application.setUserAgentStylesheet(getClass().getResource("Style/style.css").toExternalForm());