Only really useful if there are multiple users on the computer, and you the admin want them all taking advantage of this. On the other hand I can have my main user with the changes, and have test users without them, so I can see any GNOME applications (say developed in vala) naked.
I find the following works best for me.
Add the following line to my bash-profile
export LD_PRELOAD=/usr/lib/libgtk-nocsd.so
run the following in a terminal
flatpak override --user --filesystem=/opt/gtk-nocsd/ --env=LD_PRELOAD=/opt/gtk-nocsd/libgtk-nocsd.so
add the following to ~/.config
environment.d
└── LD_PRELOAD.conf
with the following content in LD_PRELOAD.conf
LD_PRELOAD=/opt/gtk-nocsd/libgtk-nocsd.so