Apliquei algum tema logo após instalar o Ubuntu MATE 16.04.
Agora o atualizador de software não aparece quando chamado no menu.
Isto é o que diz no terminal:
(update-manager:5977): Gtk-WARNING **: Theme parsing error: gtk.css:27:35: Junk at end of value
(update-manager:5977): Gtk-WARNING **: Theme parsing error: gtk.css:40:48: Junk at end of value
(update-manager:5977): Gtk-WARNING **: Theme parsing error: gtk.css:48:46: Junk at end of value
(update-manager:5977): Gtk-WARNING **: Theme parsing error: gtk.css:59:58: Junk at end of value
(update-manager:5977): Gtk-WARNING **: Theme parsing error: gtk.css:70:46: Junk at end of value
(update-manager:5977): Gtk-WARNING **: Theme parsing error: gtk.css:81:58: Junk at end of value
/usr/bin/update-manager:28: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
from gi.repository import Gtk
/usr/lib/python3/dist-packages/UpdateManager/UnitySupport.py:29: PyGIWarning: Dbusmenu was imported without specifying a version first. Use gi.require_version('Dbusmenu', '0.4') before import to ensure that the right version gets loaded.
from gi.repository import Dbusmenu, Unity
/usr/lib/python3/dist-packages/UpdateManager/UnitySupport.py:29: PyGIWarning: Unity was imported without specifying a version first. Use gi.require_version('Unity', '7.0') before import to ensure that the right version gets loaded.
from gi.repository import Dbusmenu, Unity