There are two useful techniques for figuring out the style classes for individual parts of complex components like this:
- Extract the default style sheet, modena.css, from the jfxrt.jar file and see how the default styles are defined (you can also find modena.css online)
- Use ScenicView to inspect your application when it's running
In this case I used ScenicView; the css you need is
.split-menu-button .arrow-button {
-fx-background-color: yellow ;
}