I think that the package kubuntu-default-settings installs /usr/share/kubuntu-default-settings/gtk2-engines-qtcurve.rc.sh and copies it in $HOME/.kde/env. This script contains "export GTK2_RC_FILES=$HOME/.gtkrc-2.0-kde4". A new installation of Lucid did not present the issue.
I think that the package kubuntu- default- settings installs /usr/share/ kubuntu- default- settings/ gtk2-engines- qtcurve. rc.sh and copies it in $HOME/.kde/env. This script contains "export GTK2_RC_ FILES=$ HOME/.gtkrc- 2.0-kde4" . A new installation of Lucid did not present the issue.