Take this with a grain of salt since I'm a native So Cal'er. We like the chance to wear our winter Disney gear! You will find us with hockey jerseys, hats and gloves at night. How much I have on under the hockey jersey will depend on the temp. Sometimes just a long sleeve shirt, sometimes a long sleeve, a t-shirt, and a hoodie. We always just bring everything and put it in a locker then go back as we get colder and put more layers on. I don't like to be too bundled up so I can't move when we're there so I'd rather put more layers on than one giant winter coat. Don't forget about your feet. It's really no fun to be walking around with frozen feet.
If it's 70 during the day, it's probably mid 50's at night, that's pretty chilly to us. But we've definitely been there when it's much colder.