The method Scene.getStylesheets().add(String sheet) fails in OSGi environment as it runs in a classloader different from the bundle one (which calls the method).

The API should provide at least a Classloader in the signature or better a resource provider interface.
Scene scene = new Scene();
scene.getStylesheets().add(String sheetName, ClassLoader loader)

a workaround exists but cannot load images and subfiles in the css. This workaround consists in creating a temporary css file.