STYLESHEET_CASPIAN
was introduced in FX8 (jdk8). If you intent to use FX2.x you don't need to change user agent stylesheet (caspian is only and default).
If your project compiles, but NB shows warning it's NetBeans issue. Most probably you had FX2.x in you project previously and NB remembered it. Try to change platform in project options back and forth to reset NB cache.