I live in Naples year around. From January through April you could experience a handful of days where it is in the thirties or forties overnight but will be in the sixties to seventies during that day. There also will be some days where you have a brisk wind that will make it feel cooler than it really is. There will be very little rain during that period and can get very warm going into April.
Typical summer clothing for the rest of the country, accompanied by a light jacket or sweatshirt will suffice. We have some of the mildest winter weather anywhere in the world and Naples is heavily populated by people from all over the world during that time period.