Is the implementation of any law not mentioned in the Qur’an and Sunnah considered disbelief (kufr)?

A distinction must be made between a man-made system that necessitates ruling by disbelief (kufr) and a system that does not. The administrative system that regulates and perfects matters in a way that does not contravene the Sharia, such as organizing employee affairs and managing businesses, is permissible. Indeed, Umar ibn al-Khattab, may Allah be pleased with him, implemented such systems. As for a Sharia-defying system, such as claiming that preferring the male over the female in inheritance is unfair, or that polygyny is oppression, or that stoning and amputation are barbaric acts, then ruling by it is disbelief in the Creator of the heavens and the earth, and a rebellion against the system of Heaven.