I don't know much about st terminal. But to change anything on it, you have to edit its config.h file and build from a port with a command like
(after changing into the port directory)
make ST_CONF=<path-to-your-customized-config.h> install clean.
You probably already know that, but never hurts to make sure.
I am a bit confused by your post. Are you running st on FreeBSD and ssh-ing into a Devuan machine? Or are you on Devuan and ssh-ing to a FreeBSD machine. I would think that the st you have to edit will by on the machine you are typing from.
Also, I've found that the folks at suckless are open to answering questions. You might try emailing them for help, or going on irc. (On Freenode there's a #suckless channel.)
