There is specific wording in the GPL covering that case.
Which part of the GPL do you have in mind?
Each theme is its own piece of blogging software that uses Wordpress to abstract away common problems. That's a library.
That's a library under a tremendously contorted definition of library. As I said, if that's the criterion for something being a library, any program that loads and runs extensions is a library. Firefox is a library, Apache is a library, etc. It may be your preferred definition but it's at odds with both common sense and the way the Wordpress people describe their product.