According to this Link javaFX 8u45 spinner, can be styled in numerous ways via style class. I do know how to do it by code.
For example:
spinner.getStyleClass().add(Spinner.STYLE_CLASS_SPLIT_ARROWS_HORIZONTAL);
or,
spinner.getStyleClass().add("split-arrows-horizontal");
However, not by fxml. I did try this, but it did not work. I did this via scenebuilder, version 8.
<Spinner fx:id="spn" editable="true" styleClass="split-arrows-horizontal"/>