What is the meaning of "approving of sin" that is considered disbelief (kufr), and what is the ruling on praising sin, such as saying "theft is good" or "fornication is enjoyable," and what is the ruling on showing gratitude for sin, such as someone thanking another who offered him alcohol?

Approving of sin is believing it to be good, and this is not permissible Islamically. Rather, it is obligatory to detest it in one's heart and forbid it. Praising forbidden things and glorifying their perpetrator is a grave misguidance. Denouncing with the heart is an individual obligation upon every accountable person, and it entails hating and detesting the evil, and separating oneself from its perpetrators when unable to denounce it with the hand or tongue.