How do I respond to a Christian who said to me, "Ramadan Kareem to you," and what is the proper way to respond to Christians and congratulate them on our occasions in general?

A Muslim responds to the felicitations of a non-Muslim with a greeting equivalent to theirs, or better, while avoiding what is inappropriate for a disbeliever, such as supplicating for their forgiveness. It is permissible to supplicate for their guidance, bodily health, well-being, and the like. If a Dhimmi greets a Muslim, the Muslim responds with a greeting equivalent to theirs, or with something other than "peace" such as "May Allah guide you" or "May Allah bless your morning." When responding to the phrase "Ramadan Kareem," a Muslim can reciprocate, intending that Allah include the non-Muslim in His mercy during Ramadan and guide them to Islam.