I agree with the other posters. I think thermal will probably be too warm for the bottom layer. I would wear a regular tee-shirt as my bottom layer. I also think it depends a little bit on where you're from (i.e. how cold you think "cold" is). I'm from the south (GA), so I don't wear shorts when people from up north might think it's pretty warm, LOL! December weather at WDW is very volatile, so I try to pack lots of clothes so I'm ready whether there is an exceptionally warm day or a very chilly one.