It could be. I do not really know how notification-daemon works (with gtk-2.0 and gtk-3.0 themes)
Interesting question would be, if notification-daemon works with only-gtk-2.0 version of "Lubuntu-default" theme. If it does, the problem could be related to gtkrc-file of other themes. If not, it could be an incompatibility of notification-daemon with gtk-2.0 themes.
It could be. I do not really know how notification-daemon works (with gtk-2.0 and gtk-3.0 themes)
Interesting question would be, if notification-daemon works with only-gtk-2.0 version of "Lubuntu-default" theme. If it does, the problem could be related to gtkrc-file of other themes. If not, it could be an incompatibility of notification-daemon with gtk-2.0 themes.