I just found a note from several years back. Since it may someday be handy to someone, I'm posting it here:
KDE somewhat integrates GTK applications. These applications will get the color schemes from KDE. Sometimes fonts in menu's get rendered unreadable by this.
Wrong styles are defined in /opt/gnome/share/themes/Qt/gtk-2.0/gtkrc:
class "GtkWidget" style "default"
Disabling this whole block fixed my problem. © GeekLabInfo GTK and KDE: unreadable fonts is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info