Thanks, that’s great to know, then I know I should keep trying 
Ah, well, I tried an in-place upgrade from Debian 10 before as well, and changed to a fresh template to see if that would help 
… so the problem has to be something else. Hmm.
(Btw, also now confirmed that file is actually read, with strace:
$ strace gedit 2>&1 | grep settings.ini
access("/usr/share/gtk-3.0/settings.ini", F_OK) = -1 ENOENT (No such file or directory)
access("/etc/gtk-3.0/settings.ini", F_OK) = -1 ENOENT (No such file or directory)
access("/etc/xdg/gtk-3.0/settings.ini", F_OK) = -1 ENOENT (No such file or directory)
access("/home/user/.config/gtk-3.0/settings.ini", F_OK) = 0
openat(AT_FDCWD, "/home/user/.config/gtk-3.0/settings.ini", O_RDONLY) = 10
)