The same thing you feed them in the summer, a balanced diet with the appropriate nutrients. They'll probably eat more because they need extra energy to generate heat, but they will do that from a regular balanced diet. Some people will tell you to add a lot of different things in the winter. As long as you don’t go overboard and give them enough of those extra things to upset the balanced part of their diet, treats usually about 10% of their overall diet, those things won’t hurt them. Many people are convinced they help or they wouldn’t be doing it.
It is instinctive for hens to molt when the days get shorter and stop laying. They use the nutrients that were going into egg production to grow new feathers. Pullets that started laying late summer/early fall often skip the molt their first winter but after that they will almost certainly molt when the days get shorter unless you provide lights to extend the daylight hours. Even then production often slows because of the cold weather.