Raymond said "The contents of the file aren't documented because they are changing radically in Longhorn".
While that is true, I'm writing an application with custom controls
today. Longhorn, won't ship for another 2-3 years and it might be 2009 before its widely adopted, so I think for the next 5 years documentation and a managed code wrapper are warranted. I would be happy if the documentation warned us Longhorn would change the theming completely.