From the minecraft forums:
(Sorry, my english is poor)
In biome configuration is possibility to set a sky color.
However horizon's color is always same as vanilla(light blue). I suggest to add a option to set horizon color like sky color. Anyway, this is very good mod, better than BiomeTweaker mod.
TAKE THAT, BIOMETWEAKER! . I haven't played around with sky colors yet myself, anyone with experience have some input on this? Is this a feature ppl would like to see or can this be done already somehow?