Good question because it is NOT okay to treat others poorly, so why say that? Here are some other options:
1) "Thank you." You're thanking them for recognizing that they hurt you or acted poorly. You're thanking them for making a sincere apology. You are not excusing the behavior, however. This is the most empowering choice to me.
2) "I accept your apology. Let's move on" or "I forgive you," but only if that's truly the case. If not, then state the opposite and discuss why. If you still need time to process the situation, you can say that as well. Avoid saying that all is forgotten because that is hyperbole at best.
3) If you doubt the sincerity of the apology, you can reiterate how their behavior impacted you. For example, a pseudo-apology may contain words such as "If I said," "If you thought/felt," or "If I did anything." Clarifying that yes, they did say/do something and that yes, you did consider it offensive gives them the chance to understand the impact of their behavior, own it, and have an apology redo on the spot.