Wear 3 layers of clothes when travelling outdoor is good enough: thermal wear/t-shirt, a sweater/pullover, a winter jacket. Down jackets are great. However they can be hard to store. Get gloves and thermal socks as well. Try rolling up the clothes for packing. Temperature will drop beyond 0 deg cel in Dec.
Toiletries and Medical Supplies: Bring and apply sufficient moisturizer for the skin in the cold weather. Try Vaseline. I got peels when I returned to S'pore as I didn't protect my skin well.
Clothing/Shoes/Weather Gear: I live in the neighbor country, Norway, but I still found it freezing in Stockholm.. It may have to do with the icy breeze coming in from the sea.. I would pack lots of warm clothes; jacket, sweaters, thick socks, good shoes, gloves/mittens, hats/ear-muffs, scarves..
Don't forget a couple of warm jumpers, a coat and a scarf. Winter can be very, very cold. A good pair of gloves will be appreciated as well.
Photo Equipment: You will find everything in Stockholm but it might be expensive.
If you go to Sweden in the winter, as I did, bring plenty of long underwear. Dressing in layers is best, and I found a couple layers of thin cotton long underwear to keep me very warm without bulk. Also, wear a hat and scarf in winter to keep you warm. Your health is more important than how you look.
Toiletries and Medical Supplies: Sweden is a very modern country so I wouldnt worry about running out of any toiletries, whatever you need should be easily available.
Sturdy shoes with a lot of grip - while the pavements are gritted and cleared, sometimes they can be very icy. Hat, scarf and gloves are essential - try Stadium for cheap sports gloves, hats, scarves otherwise there are lots of designer places with very expensive ones!
Toiletries and Medical Supplies: hand creams, moisturisers
Photo Equipment: quick lenses - light is not good before 9am and after 3pm.
Bring the warmest cloths you have and it still won't be enough :)
Toiletries and Medical Supplies: Non needed
Photo Equipment: Oh yes! Bring your camera!!! Stockholm is amazing during wintertime!!!
Camping/Beach/Outdoor Gear: I wouldn't recommend camping during winter time ;-)
Clothing/Shoes/Weather Gear: If you're travelling during the winter you'll definitely need your big fury hat and gloves! This photo was taken in January when it was MINUS 20 degrees celsius!!! FREEZING COLD!!!