Selecting font for the whole buffer?

My font setting code in

https://github.com/ngtk3/NEd

is based on

https://developer.gnome.org/gtk3/stable/ch01s04.html#id-1.2.3.12.5

In the last days I added split views by using GtkPaned, which was easy
and works fine. But I discovered that when inserting text at the
beginning or end of a buffer, that text gets not the selected font, but
the system default font. Not very surprising for me, from the beginning
I wondered if that may occur. But I have currently no idea how to fix
that. The solution should be hidden in gedit code of course, but I have
no idea for what I should search, so it may take many hours. Maybe I
should wait for some dark winter days...