Webspark could def. use some sweet love!
For a desktop Spark, JavaFX is the way to go here. JavaFX is the successor of Swing and is standard in Java8, but you can already use it with java7 (you just have to bundled the fxrt.jar with your project, i think new java7 releases may come with that lib, but just not loaded in the classpath).
JavaFX uses CSS natively for styling, can use FXML for a very easily building a UI layout (even non-programmers can hack on the FXML, it’s an XML with special nodes).
Also it retains a 100% Java Spark.
The only problem is retro-fitting Spark to use JavaFX instead is a large task in itself…
(disredarg anything < JavaFX 2.2, especially JavaFXscript, > JavaFX 2.2 has been standardized and is fantastic)