Skybox Basic Details
The easiest way to use a skybox in The Dark Mod is to use the prefab ones which come in the resources.
The best way to get good performance out of the Dark Mod is to create corridors that are visportaled off from eachother.
Therefore the outside area of your building should be a series of corridors around the building with the brushes designated skybox stretching from the outside wall or buildings forming the border of your level, to the highest point of the building you want to be visible, so the top of the roof.
This enables the use of visportals stretching from the skybox to the ground at key corners so that when you are standing on the north side of the map you won't be rendering the south side at the same time.
The outsides of the brushes which face outside the level have to textured with anything that is solid and a valid texture, people often use caulk, but as long as it's not No_draw you're fine.
Texture the inside facing sides with the textures/textures/smf/portal_sky texture, ensuring you only texture the inside facing sides, selecting them with control shift click.
Go to the File menu, then “Load prefab” and navigate to the “prefabs” folder in “darkmod” folder and select “tdm_sky_starry1.pfb”.
Lock textures or the moon texture may get out of alignment.
Place this outside the main hollow box of your level. It is caulked on the outside and sealed so it will not leak.
Finished. When you load your level you will have an attractive night sky with clouds moon and stars.