I rebuilt the Niflheim tileset using textures found in the SC2 data files. That's right - the textures for a snowy planet were there all along. What is absent, however, is supporting doodads. In this demo map I've placed many doodads so you can judge whether or not the lack of snow-covered rocks and other supporting doodads is a dealkiller.
Feel free to use this map as the basis for your own projects.