It's rather complicated, but I managed find out how you do it. Here's an
outline of the steps:
Fix ownership of directories used by gdm.
Start a dbus session for gdm and remember it's address and PID.
Configure parameters of gdm, such as the font.
Kill the dbus session.
Restart gdm.
Fix the ownership of directories for GDM, otherwise you will not have the
needed permission to configure anything:
sudo chown -cR gdm: /var/lib/gdm/.config
Start a D-Bus session as GDM, so you can use gsettings to configure it. Also
copy the output of the command:
sudo -u gdm dbus-launch
The previous command will output stuff. You should save the two lines
similar to the following:
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-hbrQyUX0jK,guid=83294e9d35f2e0c5a969fcd9000007ad
DBUS_SESSION_BUS_PID=1234
Now you can set the font size of GDM. It's done with one long command. You
use the "...ADDRESS=" from previous output. In my example I set the font to
"DejaVu Sans 9":
sudo -u gdm
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-hbrQyUX0jK,guid=83294e9d35f2e0c5a969fcd9000007ad
gsettings set org.gnome.desktop.interface font-name 'DejaVu Sans 9'
You can set other things with gsettings, such as the icon theme. When you're
ready as good practice, you should close the D-Bus session you've started.
You use the "...PID=" line from before:
sudo kill 1234
If you want to see what you've done now, you can restart GDM. Keep in mind
that this command will probably close all you programs:
sudo restart gdm
Thats's it.
I've also wrote a script for this:
#! /bin/bash
# Give GDM ownership to its files
sudo chown -cR gdm: /var/lib/gdm/.config
dbus_address=""
dbus_pid=0
# Start a D-Bus session for GDM and save its address and PID
for var in $(sudo -u gdm dbus-launch); do
echo $var
if [ $(echo $var | grep -E '^DBUS_SESSION_BUS_ADDRESS=') ]; then
dbus_address=$(
echo $var |
sed -r 's/^DBUS_SESSION_BUS_ADDRESS=//'
)
elif [ $(echo $var | grep -E '^DBUS_SESSION_BUS_PID=') ]; then
dbus_pid=$(
echo $var |
sed -r 's/^DBUS_SESSION_BUS_PID=//'
)
fi
done
# Set GDM settings
sudo -u gdm DBUS_SESSION_BUS_ADDRESS="$dbus_address" \
gsettings set org.gnome.desktop.interface font-name 'Sans 9'
# Close the D-Bus session
sudo kill $dbus_pid
