I think I answered this before when you asked this same weather question on another thread (about New Year's weather, maybe?). But in addition to what Amamax2 said, which is accurate, our 30 degrees in SoCal (if that's what it drops down to) is not going to feel the same as the 30 degrees to which you are accustomed in Chicago. Chicago is more biting, bitter cold in winter...often with snow. If it is not raining in December - and it could be - the weather in SoCal in the daytime may be cool, but it will seem warmer in DLR. And when it gets cold at night, there will be no snow. It may require a jacket and some gloves, but because you are used to Chicago weather I don't think you will find SoCal nearly as chilly, even if the temps drop to 30 degrees.
I've been to DLR in December when it rained. I've been there in December when it was 70 degrees in the daytime and 50 at night, and I was running around in a t-shirt the entire time (no jacket) but was not cold at all. I've been there when it was only in the 40's in the daytime and then 30 degrees at night and I was freezing, bundled up with mittens and layers and all kinds of things. It's all over the map, really.