What is the theme in The Concept of Law by H. L. A. Hart?

Formalism and rule-skepticism is another theme in this book. Hart is also famous for recognizing that primary and secondary rules can be vague at their edges and cannot of themselves determine how they are to be applied and interpreted. Neither can other rules determine how they are to be applied. Language itself possesses sloppiness and ambiguity that always leaves room for reinterpretation. Hart thus says that law has an "open texture".