in that case you need to mount a file to your container
inside the sogo-app systemd service add
--volume %S/state/custom_theme.js:/usr/lib/GNUstep/SOGo/WebServerResources/js/custom_theme.js:Z
custom_theme.js must be in the state of the user /home/sogo1/.config/state
few chances that we add it to everybody
then if you want to be persistent to updates, create a custom systemd service
systemctl --user edit sogo-app