Set of commands and files to enable timed theme (dark/light) change

theme_switcher.sh

kretyn@meland:~/$ mkdir -p ~/.config/systemd/user
kretyn@meland:~/$ mkdir -p ~/.scripts
kretyn@meland:~/$ cd ~/.config/systemd/user

kretyn@meland:~/.config/systemd/user$ cat light.service 
[Unit]
Description=Automatically change the "Window Theme" to "light" in the morning.

[Service]
Environment=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
ExecStart=/home/kretyn/.scripts/profile_changer.sh light

kretyn@meland:~/.config/systemd/user$ cat light.timer 
[Unit]
Description=Automatically change the "Window Theme" to "light" in the morning.

[Timer]
OnCalendar=*-*-* 06:00:00
Persistent=true

[Install]
WantedBy=default.target

kretyn@meland:~/.config/systemd/user$ cat dark.service 
[Unit]
Description=Automatically change the "Window Theme" to "dark" in the evening.

[Service]
Environment=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
ExecStart=/home/kretyn/.scripts/profile_changer.sh dark

kretyn@meland:~/.config/systemd/user$ cat dark.timer 
[Unit]
Description=Automatically change the "Window Theme" to "dark" in the evening.

[Timer]
OnCalendar=*-*-* 19:00:00
Persistent=true

[Install]
WantedBy=default.target

kretyn@meland:~/$ cat ~/.scripts/profile_changer.sh
#!/bin/bash

get_uuid() {
  # Print the UUID linked to the profile name sent in parameter
  local profile_name=$1
  profiles=($(gsettings get org.gnome.Terminal.ProfilesList list | tr -d "[]\',"))
  for i in ${!profiles[*]}
    do
      local uuid="$(dconf read /org/gnome/terminal/legacy/profiles:/:${profiles[i]}/visible-name)" 
      if [[ "${uuid,,}" = "'${profile_name,,}'" ]]
        then echo "${profiles[i]}"
        return 0
      fi
    done
  echo "$profile_name"
}

if [ $1 == "dark" ]; then
    THEME='Yaru-dark'
elif [ $1 == "light" ]; then
    THEME='Yaru-light'
fi
UUID=$(get_uuid $1)

/usr/bin/gsettings set org.gnome.desktop.interface gtk-theme $THEME
/usr/bin/gsettings set org.gnome.Terminal.ProfilesList default $UUID


kretyn@meland:~$ systemctl --user daemon-reload 
kretyn@meland:~$ systemctl --user enable dark.timer light.timer
kretyn@meland:~$ systemctl --user start dark.timer light.timer 

It took me a while to figure it out, but you can add this to change all currently running terminal windows to the correct theme also:

gdbus introspect -e -x -d org.gnome.Terminal -o /org/gnome/Terminal/window |
  xmllint --xpath '/node/node/@name' - |
  sed -e 's/name="\([0-9]*\)"/\1\n/g' -e 's/ //g' |
  while read -r __node; do
    gdbus call -e -d org.gnome.Terminal -o /org/gnome/Terminal/window/${__node} -m org.gtk.Actions.SetState profile "<'${UUID}'>" "{}" >/dev/null
  done

Note that this only changes the active terminal tab.

Neat @W-Floyd, Thanks! :)

I had problem to execute this but it turned out that the xmllint isn't installed by default. Needed to install libxml2-utils (sudo apt install libxml2-utils).