How can I change the default properties of objects such as a textbox that are not present on a slide layout?

I am using Keynote on a MacBook. When I add a textbox to a slide it has a font style which I don't like, so I have to change it manually every time. Can I change this in the slide layout? Of course I can add a textbox with the right properties to the layout, but then every slide that I create has this textbox. And when I insert another one it still the same undesired properties. The same holds for other objects that can be inserted: I would like to be able to change their defaults too.