Toronto is cold in Winter, but it is located at 43.7 degrees Northern Lattitude, which is similar to the lattitude of Marseille in Southern France. So, you can hardly say that this is a dark place in Winter. I think the main problem is that in Winter the days are a bit shorter so it may still be dark when you go to work and when you're back it is already starting to get dark. You can then get some sunlight by going outside during the lunch break.
You also mention the cold, but Toronto isn't so extremely cold that going outside would be a challenge even when properly dressed. This this means that you are not wearing good winter clothing and then suffer as a result of that. This makes you avoid going outside, which leads to the winter blues, which causes you to become reticent to going outside even more. And, of course, if you feel like going to sleep, you may start to feel a bit chilly even while sitting in a properly heated room; the thought of going outside when it is -20 C alone may then make you stay indoors.
You have to stop associating the feeling of cold with the outside conditions. What you feel is your own body heat, this is regulated by your own body thermostat. Simply dressing properly will allow you to use your own body heat to stay warm outside.
The basics of how to dress well is quite simple:
The key thing is to wear multiple layers, a lot of air will be trapped in between the layers which will give a good insulation against the cold. In the picture the girl is wearing rather light clothing appropriate for physical exertion in cold conditions. If you're going to be doing only light exertions, then the thermal underwear must be thicker different, e.g. you can use this. If it is colder than about -25 C then ski trousers won't do, you must wear down filled trousers. You may also need a wind stopper on top of that. The hat you wear cannot be just any ordinary hat, if it is cold you need to wear an Arctic hat like this:
In extremely cold conditions, face masks like this can keep your entire face warm and moisten the air you breath in using the moist in the exhaled air.
Finally, the boots should be of good quality, the best boots have multiple layers with removable inner boots, these are capable of keeping your feet warm at temperatures as low as -40 C.