I've noticed this for a while in the 6.0.x builds and am surprised that it doesn't seem to have been fixed, even in the latest 6.0.2 beta builds. Using Windows XP look and feel (any theme) and Java 6 (any recent build, I'm using 104), most message dialogs have a white background behind the text. See attached screenshot.
It looks a bit scruffy really and shouldn't be too difficult to fix, I hope. It would help polish the UI!