WORKAROUND: configure gtk3 in the home directory of lightdm user
/var/lib/lightdm/.config/gtk-3.0/settings.ini
with following text:
[Settings] gtk-theme-name = Ambiance gtk-icon-theme-name=ubuntu-mono-dark gtk-fallback-icon-theme=ubuntu-mono-dark
so somehow the lightdm user environment seems not configured properly. If this the reason, on my box without this annoyance there should be a more complete set of config files. I can look for it on monday.
WORKAROUND:
configure gtk3 in the home directory of lightdm user
/var/lib/ lightdm/ .config/ gtk-3.0/ settings. ini
with following text:
[Settings] theme-name= ubuntu- mono-dark icon-theme= ubuntu- mono-dark
gtk-theme-name = Ambiance
gtk-icon-
gtk-fallback-
so somehow the lightdm user environment seems not configured properly. If this the reason, on my box without this annoyance there should be a more complete set of config files. I can look for it on monday.