/usr/share/themes/Default/xfce-notify-4.0/gtkrc is in xfce4-notifyd 0.2.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # this is deliberately empty: this theme will use whatever default colors
# are in your main gtk theme
|