This was achieved using devilspie to start darktable maximised, followed by my linked answer to remove the window decoration.

Note - I tried this with the devilspie "undecorate" command but that just left an annoying blank space where the decoration would be

install devilspie

sudo apt-get install devilspie

mkdir ~/.devilspie

nano ~/.devilspie/darktable.ds

copy and paste the following:

  (is (application_name) "darktable")

    (begin

       (maximize)

       (spawn_sync "/home/dad/decoration.py")

       (spawn_sync "/home/dad/decoration.py")

    )

N.B. substitute /home/dad/decoration.py for the full-pathname to the undecorate script. Save.

Finally, make sure devilspie start in your startup-applications.

Linked Question:

1. Remove titlebar from maximized terminal window