ANSWER:

This is not kufr.

However, one must refrain from doing this as it might eventually lead him to sing it verbally and then satan would make him listen to songs.

Enjoying such kufr words indicate that there is a seed of hypocrisy in his heart that is growing strongly. 

If a person plays/sings songs in his mind which has words of Shirk and Kufr but he does not say it with mouth only that song plays in his mind will this be considered as minor or major shirk or kufr or will he be sinful? Please specify.