If you don't have experience and gear that you can trust in the winter, either stay very close to an open hut or maybe even better, stay in the forest.
The mountains above the tree line in winter is a very unforgiven place. 500m distance to get to safety might prove much longer than you could imagine when there is a lot of snow and nothing to protect you against the wind.
Most people have either lots of experience or at least gear made for winter camping. With a winter tent, winter clothes, winter cooking equipment and so on.
In the forest you will probably do fine even if you don't have all the right stuff or make a few mistakes. Also it is easier to get a cozy fire going.
The reason the huts are closed in December is that most people choose to go camping later in the winter or the winter/spring, just like
@packåsna writes. But there are many STF huts open in december, especially in the southern parts of the mountains.
If you want to have a town or village nearby, maybe check the woods in Dalarna? Lots of cozy villages, lots of woods. Looks like they will have some snow in 2 weeks but nothing crazy.
Here you can find the official map of sweden:
https://minkarta.lantmateriet.se/
To find places in the woods, look for the areas inside of green lines. That is protected areas, and many times the nature is nicer in those places, no heavy foresting or stuff like that.
If you find something that looks good, google the name of the protected area + länsstyrelsen
That way you find the information and rules for the area. The rules is usually in a bullet list in the bottom of the page. Use google translate or something to find out if they allow camping in the area or not. There will also be information about possible fire ban in the area.
There is a mail adress you can write to if you have questions.
You can also find information about hostels and areas and supermarkets and open huts here:
https://naturkartan.se
If you choose to camp in the mountains at this time of the year, you could either camp outside of an STF hut (at least in the southern mountains (ie Härjedalen and Jämtland) you will find open places. You pay a small amount of money / night to stay close to the STF hut and can use their toilets and showers and open kitchens for cooking.
You could also use the map naturkartan to find the small open huts of Länstyrelsen. They are marked with a symbol of a house at naturkartan.
Those small open huts are for breaks. You are not allowed to sleep in them and not allowed to use the firewood to heat them, if you're not in an emergency. But its perfectly cool to go inside for a break. If there is other people that want to have a break you take turns.
Camping close to a hut like that will be pretty safe.
Whatever you choose to do, if you will depend on that some place is open, call them or mail them and make sure that it will work this year with corona restrictions.