Customizing widgets with custom themes

Has anyone found any documentation on creating your own themes for the Widgets 2.0?

I have found the “SetTheme” API here :http://docs.coronalabs.com/api/library/widget/setTheme.html

But it doesn’t talk about how you can supply your own images for a theme you would like to create?

Any direction on how to visually change the widgets 2.0 would be wonderful!