In Mate there’s another command, that needs dbus-launch to be launched as root.

For root, I always set fonts, colors, icon and gtk themes, sound events different from those for a regular user. It helps to be aware when I’m running something as root.

In Mate it was usually done with

gksu mate-appearance-properties

It doesn’t work correctly anymore, and should be launched as

gksu dbus-launch mate-appearance-properties