Own drag icon with same color and font settings as the default drag icon in a Gtk.TreeView

[<c buhtz posteo jp>]

I have a stackoverflow question with example code open with this:
https://stackoverflow.com/q/52339893/4865723

The Gtk.TreeView implements a default drag icon. It use the background
color of the TreeView, it's font and the complete row-content as string.

I want the same (background-color, font-face, font-size, font-color)
but with a shorter string (only the second of three columns).

How does the TreeView get this values?