ScrollBarSkin is a class representing the skin used to render the ScrollBar. Here is an extract from a default JavaFX style sheet:
.scroll-bar {
-fx-skin: "com.sun.javafx.scene.control.skin.ScrollBarSkin";
}
Here is a link to ScrollBarSkin.java in the JavaFX 8 source repository. Note that it is a com.sun class, so it is not part of the public API and could disappear or change API between minor JavaFX releases without notice.
You can override the default skin with your own skin via the following css in your user stylesheet:
.scroll-bar {
-fx-skin: "com.mycompany.control.skin.CustomScrollBarSkin";
}
I just made the name and path up, you can use whatever you want.
What the skin is allowing is programmatic control over the look of the a control (i.e. it's only incidentally related to css because css is one way to set the skin on a control).
Customizing Skins is documented (to a certain extent) in the OpenJFX wiki.
The skin customization relies on a new JavaFX 8 class called SkinBase, which forms part of the javafx.scene.control public API.
Customizing skins in versions lower than Java 8 is not recommended, because then you will be working with old, undocumented and unsupported private APIs which will not work with Java 8 and later. Customizing skins in Java 8 is fine because it relies on the public API.
I'm pretty sure from your question that this isn't really what you are looking for, but it is the answer to your question (at least as I understood it).