Similar questions to this are asked periodically, but many of these answers are outdated.
I need to build a cross-platform desktop application in Java with a GUI of comp
Modern Javascript frameworks (ExtJS, Dojo, etc...) offer the same widgets richness or more (Excel like grids for instance), a wider variety of L&F and usually fit better with the OS of the user. Users are also very comfortable with their browsers and, hey, "modern stuff" is on the web, and the modern web today is HTML+Javascript.
The overhead of converting an app to "web like" is minimal. An embedded Jetty can remain really small and disk space has become much less of an issue.
There are additional benefits going down this route for the future evolutions of the application.
EDIT
3 years later this has become extremely easy thanks to Electron
I know this question is old, but if you don't want to use FX and still want to use Swing, then try MacWidgets, i've used it on a couple projects. It's very light, and looks great. Below is an old project i was working on, over time i've perfected using macwidgets and now use it internally in my company.
http://www.digitalhand.net/projects/jdataanalyzer/mainGUI.png
Swing is good (stable, documented, supported until 2026). The problem is that it relies on the LookAndFeel system that provide a very limited number of boilerplate choices. It should be easy to change the appearance of each component individually and then unleash its creativity. Unfortunately it is painful. IMHO frameworks should be built on top of Swing to make it possible instead of creating JavaFX.
Take a look at https://github.com/dotxyteam/ReflectionUI.
Ex GUI: http://javacollection.net/reflectionui/wp-content/uploads/2017/08/general.png