I would suggest a thin jacket to protect you from cold wind and a thin sweater that you can put over whatever you are wearing. Both together should protect you enough. The temperature might drop end of December/January some more so the later you come the colder it gets, sometimes down to 8 degrees. Right now it's 25 for example.
What I have to tell you however is that you will never get a good answer to that. I know people from South India who wear woolen hats and winter jackets when it's below 30 degrees. And I see enough people in Hong Kong that wear shorts & t-shirts when it's 10 degrees in Hong Kong. So you will have to make your own decisions.
It also highly matters where you are going, what you do and where you stay. Feeling cold is not a matter of being outside in Hong Kong. Homes and offices are usually colder than hotels and restaurants. Homes have often very thin windows and few have heaters and offices have maybe good insulated windows but a central aircon that cannot heat and will blow cool air the whole day. That makes them colder than outside in cases. I know enough people who walk around in thick jackets in the office and sometimes at home during the days where it's below 15 degrees.
Outdoors it also depends what you are doing... hiking, walking around, sitting somewhere...