Moderns are basically no different than any other chicken if you are just keeping them around. Any commercial grade ration will be just fine.
If you are breeding them right now then I would use a little higher protein feed. I use a 20% game bird layer as my basic feed for breeding and conditioning for show. Once or twice a week I will feed soaked whole oats to help make them lay and it also hardens the feather quality for the show room.
As a starter I have used a 28% medicated feed for a long time because I also have peacocks and guineas. But I have recently gone to a 20% starter because my moderns were getting a little long in feather because of the high protein.
Good luck
Bob