After a little bit of experimentation with the SC2 Editor, I have found a way to have a sky in the editor in a tileset other than Braxis Alpha.
The technique is simple, although it doesn't always work for me, so it's best to take every precaution. Start your map with the Braxis Alpha tileset. Make sure that the Braxis Alpha sky loads properly. Next, make a small mark with each terrain texture for Braxis Alpha in a corner in of the map. Set up a camera that allows you to view the sky. Go to Map -> Map Textures. Select the tileset of your choice, set up your texture replacement as you wish, and push okay. If all goes well, you should now have a completely new sky for your map.
There are some tilesets which still do not have any skies (Korhal, Monlyth, Redstone, and Tarsonis.) However, all other tilesets do have skies. Shown below is a terrain using the Mar Sara sky. Unfortunately, this method only allows you to use the sky which corresponds to the texture set you are using.
Enjoy!
The technique is simple, although it doesn't always work for me, so it's best to take every precaution. Start your map with the Braxis Alpha tileset. Make sure that the Braxis Alpha sky loads properly. Next, make a small mark with each terrain texture for Braxis Alpha in a corner in of the map. Set up a camera that allows you to view the sky. Go to Map -> Map Textures. Select the tileset of your choice, set up your texture replacement as you wish, and push okay. If all goes well, you should now have a completely new sky for your map.
There are some tilesets which still do not have any skies (Korhal, Monlyth, Redstone, and Tarsonis.) However, all other tilesets do have skies. Shown below is a terrain using the Mar Sara sky. Unfortunately, this method only allows you to use the sky which corresponds to the texture set you are using.
Enjoy!