Edit an existing keynote theme

Is there a way to edit and then save an existing Keynote theme. I've created a custom theme but now I want to make some changes to it. It seems the only way to do that is to open a new presentation with the current theme, go to Format side bar, select Edit Slide Layout, make the changes, and then save the changes in a new theme. This seems like a roundabout way of doing it, especially as i want to make changes the theme, see how they look in a test presentation, and then go back and make more changes or undo what I did if I don't like. This means I end up with multiple old versions of the theme that I then need to go back and delete!

Is there an easier way to do this?