What is the ruling on congratulating someone who has committed a sin, jokingly saying "Mabrouk" (congratulations), such as when someone has stolen and their friend jokingly says to them "Mabrouk"? And does this matter reach the level of disbelief (kufr)?

Congratulating a person who has committed a sin is not an act of disbelief (kufr). It appears that the questioner is suffering from excessive whisperings (waswasah) regarding disbelief. The cure for this is to disregard these whisperings, to cease dwelling on them, and to refrain from asking about them.