The screenshots are of the default widgets. The only question is which is the selected look-and-feel (for the "old" Swing toolkit), or the style sheet (in the new JavaFX).
On my Mac they look as close to native as any other app, which is to say that applications no longer have a uniform look anymore anyway. Chrome, Firefox, Twitter, my email client and all of my editors look no more "native" than modern Java apps like NetBeans or IntelliJ.
https://netbeans.org/images_www/screenshots/platform/jmonkey...
https://netbeans.org/images_www/screenshots/platform/nbAWSAd...
https://netbeans.org/images_www/screenshots/platform/lg_soft...
https://netbeans.org/images_www/screenshots/platform/GGSSEph...
https://netbeans.org/images_www/screenshots/platform/platypu...
https://netbeans.org/images_www/screenshots/platform/blueMar...
or the new Java crap like
http://fxexperience.com/wp-content/uploads/2011/05/Charts-Ba...
http://docs.oracle.com/javafx/2/best_practices/img/saleshist...
https://blog.idrsolutions.com/wp-content/uploads/2014/03/vie...
??