This is copied from Adwaita. Looks sufficiently ok.
https://bugzilla.gnome.org/show_bug.cgi?id=729346
b83880b W32: Add a theme for .app-notification
gtk/resources/theme/gtk-win32-base.css | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
Upstream: git.gnome.org