TL; DR I want to give my "snow elves" a magical ability to slow down the loss of body heat, which when used would constantly drain their mana and make their other spells weaker. It would be greatly appreciated if someone could offer another perspective so I don't miss anything important, or anything potentially interesting that I could use for building their culture, history, or the plot. If interested, I've typed everything significant I could think of, to limit redundancy.
With this technique, it would take several years to learn how to have even a rudimentary grasp of it, and would start being taught as soon as the children can understand speech. It would be always be consciously turned on or off, and after a fair bit of training and experience, could become a subconscious process like breathing. Every snow elf would be expected to have a grasp of this, with a minority of individuals having a difficult or extremely hard time learning and/or using it.
Regaining mana (not just them, everyone) would require the digestion of food or medicinal herbs/potions, with moving animals providing more mana than plants. Since they'll be living in the north, I don't think they'd grow that many crops anyway, but rather live off large furry animals and fish.
I imagine the constant need for high mana food to would encourage them to eat more meat, especially their fighters. They would split into coastal/waterborne tribes primarily fishing and inland/nomadic tribes primarily hunting, though there of course would be many tribes doing both in good proportion. I guess I could make it that the more southern tribes are more civilized and practice husbandry, raising fish farms or animal herds. Or maybe there are some tribes that mostly try and grow whatever can grow in the far north and be more efficient that way.
I can already guess that this society would follow more eastern philosophies valuing traits like strategy and wisdom, considering the limiting factor during combat. Combat would mostly rely on quickly and powerfully attacking with fire magic before running out of steam, like a blitzkrieg or something. They'd be a serious force in the north, mostly having to deal with invading humans, dwarves and giants, but mostly remaining divided among their clans.
I still think they'd wear warm clothing, though the degree could vary depending on how long an individual could use the technique. Maybe the exposure of skin outside is seen as a flex, to show off how well they can survive the cold.