Note: If you want to create a notification theme you will - due to backward-compatibility breakage - have to take into account which version of Gtk+3 the users of your theme will most likely use or have installed.
Generally speaking if you want to create a theme for Xfce4-notifyd you have to override the styles of the currently in-use Gtk theme as good and completely as you can to ensure the user-experience will be consistent with all base themes. So if for example the user uses
Adwaita (the built-in Gtk+3 default theme) you will have to at least define everything this theme has in the specific widget styles (see the list of items to match below) to ensure your style will take complete precedence.
Adwaita defines the
osd class shall have a black background and a white foreground color and you only set
background-color: #ecde00 (which is yellow) the notification messages will likely be unreadable.
For examples of themes simply check https://git.xfce.org/apps/xfce4-notifyd/tree/themes
Note: The gtk-3.0 folder is for Gtk3.18 and before.
In order for Xfce4-notifyd to find your theme and show it in the Preferences Dialog you have to save it as
gtk.css in either
/usr/share/themes/$yourtheme/xfce-notify-4.0 (for global installation) or
~/.themes/$yourtheme/xfce-notify-4.0 (for local, single-user installation).
#XfceNotifyWindow.osd progressbar progress
#XfceNotifyWindow.osd progressbar trough