com.sun.* code is not public API, which means we do not make any commitment to keep it stable between releases. We reserve all rights to change this API, which may totally break any code you write that extends from, or makes use of, these APIs. In summary, we do not condone using these APIs, and don't be surprised when the API changes in future releases if you do use them.
To answer your other question, CSS is a very powerful way of modifying the look of the UI controls. You should refer to the caspian.css file that is distributed in the jfxrt.jar file for real-world uses of CSS to style the JavaFX UI controls.