Hey, sorry for the late response!
It was a problem in the GTK2 theme. Actually, I think that's a (really odd) bug in XULRunner (which is what those programs you've mentioned use to render the user interface), since I just had to change one color for it to be fixed, lol.
Anyway, the new bugfix version is already uploaded and you can download it from the link above.
Thank you very much for the feedback, and don't hesitate to report any other bugs you find!