GtkPaned: visible handle?

[Roland Koebler <rk-list simple-is-better org>]

Hi,

according to the documentation (and the image on
https://developer.gnome.org/gtk3/stable/GtkPaned.html),
GtkPaned-widgets have a visible handle (e.g. 4 dots)
to show that the user can drag this handle.

Unfortunately, these handles are invisible here -- both
in Glade and in the running program. Is there any way
to make them visible?

thanks,
Roland