Hi Patrick,
To define a default property value, you need to specify a string with the word 'default', the object type (ex: Axes), and the property name (ex: Grid Alpha). For example, in order to set the properties you mentioned, you would do:
set(groot,'DefaultAxesGridAlpha',1);
set(groot,'DefaultAxesGridColor',[0.1 0.1 0.1]);
As of R2014b, MathWorks released a new graphics system wherein graphics handles are now objects instead of doubles. In doing so, you can more easily define properties of a particular level. In addition, the "root object" is now called "groot" (graphics root), although you should still be able to use 0 to set default values. For more information on setting default values, I recommend looking at this page.
Hope this helps!
