Because it's not a christmas season but a winter one, and we're still in winter? Well, at least in the northern part of the planet. I think the themes used for the forum are for Spring, Summer, Autumn and Winter, so the next change must happens during the month of March.
It's just general winter theme without focusing any holidays. It works while December, January, and February. On the Northen semispshare of Earth is winter (especially temperate zone, not counting polar where is infinite winter), so yea.