Comment 8 for bug 857577

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.