One problem that I believe many Swing developers are probably facing, or will be facing soon, is how to migrate existing Swing applications to JavaFX in the future. A current approach is to slowly integrate JavaFX components into an existing Swing application, but the look and feel of the components are very different. I believe that there is a need to create a JavaFX Modena theme for Swing so that mixed (Swing and JavaFX) applications can look great together.
Is this something that your new StyleManager would be able to facilitate? For a complete rundown of what the Modena theme is like, take a look at http://fxexperience.com/2013/03/modena-theme-update/.
Yes, it will be possible to add any kind of themes including the one presented on the screenshots behind the link. Though it is still up to you how detailed and precise that theme will be.
I might also be adding that theme on my own later since it is really nice one and might be useful for those who actually use JavaFX inside their Swing applications.
I also recommended to add that theme. It will be very use full also e.g. NetBeans Platform programmers that create new module using JavaFX.