You adjust your sunlight with your World Spawn Settings. It looks like your problem may be no world spawn settings or your sun is shining from under your map. Which is why it looks like that.
With nothing selected Press (N)
This brings up the entity window. Ive noticed some peoples maps not including the World Spawn settings which include things like Sunlight. Ambient Light ect. Copy one of these using each key and value of whatever one you choose, and see how it looks.
http://modsonline.com/Tutorials-read-445.html (http://modsonline.com/Tutorials-read-445.html)
If your world spawn is blank. I think would just see World Spawn in one kvp box after press N. you would just make all these entries as new. If you do have sunlight etc already just use the new numbers.
Also with your sky being black you maybe dont have a skybox. Make sure you have one. Will See ***LEAKED*** when compile if you dont have one.