Here is the solution to the very common problem of bright terrain:

1. Open the map in zero editor.
2. Go to the colors tab.
3. Darken the foreground to "128 128 128" using "Color Fill".
4. If your terrain is still bright,
make sure you have a texture detail for whatever texture is still bright (this solved the circle of light around the player for me).

The brightness of the terrain has noting to do with the sky file (afaik) so you need not change that.

NOTE: you may want to burn the terrain by going to the "image" tab in zeroeditor then selecting burn terrain. Im not sure if it is necessary.