Typically highs would be in the low/mid-60s with lows in the lower 40s at night. It's definitely not shorts weather in December for the most part, and you will want jackets for the evening.
Occasionally we get an odd hot or very cold spell that time of year but for the most part expect cool but comfortable temps.
I have been a few times in December and during the day sometimes you can wear shorts and be fine. I tend to wear pants just in case. The evenings will be cool - very cold. A couple times we had to buy an extra sweatshirt!!! It is best to be prepared. We usually dress for the day and then return to the hotel for a rest and come back with all the coats and such on the second trip over.
Lived in So. Cal. my whole life. December is typically a beautiful month, and it can even get hot. While the air temperature may be in the high 60's or low 70's the sun is usually out and it feels much warmer. You can definitely get away with shorts during the day, and jeans and a sweatshirt would be more than enough in the evenings.
Enjoy all the decorations! It's the best time of year at Disneyland!