The settings dialog can be either invoked via the application menu or the commandline:
xfce4-notifyd-config and allows you to configure the following settings.
When set to Default the
app-notification style classes of your currently selected Gtk+3 theme will be used.
When set to a different theme-name, it will use the respective standalone Xfce4-notifyd theme.
This setting determines where on your screen notifications will show up by default.
This timeout determines how long notifications will be shown until they gracefully fade out. If a notification is designed to be persistent, this value will not apply (e.g. warnings related to low battery power).
This setting allows you to set a default opacity for notifications. This only works when using a compositing window manager.
This shows a sample notification based on the style you have selected in the options described above.