Where is the documentation for the Widget "theme file" structure? Or an example?

In the documentationfor the setTheme() function it says:

 See the  WidgetDemo  sample to see how a theme file is structured and can used in a real app.

But there is no theme set in the WidgetDemo sample! And as far as I can see, no other documentation for this at all.

cornerRadius is no longer supported and I need to override the default theme to make it zero. But more than that, I’d really like to know how to tweak the theme.