GUI Skinning theme tutorial doesn't provide inheritance information

I was working through the creating a custom GUI skin tutorial, and realized there was no view of how each theme element inherits from one another. This can be figured out ultimately by looking at the list of inheritors of Control, but that is not explained. This could be a line or so addition, and would be helpful.

Example issues I encountered specifically.
* Creating a Class Item for panel, and discovering I needed to instead make one for Panel Container.
* Finding a common ancestor to set stuff like the background color of all text fields.