skyColor and skyAngle - sky gradation

groundColor and groundAngle - ground gradation

Background {
    skyColor    [ 0.1 0.1 0.0, . . . ]
    skyAngle    [ 1.309, 1.571 ]
    groundColor [ 0.0 0.2 0.7, . . . ]
    groundAngle [ 1.309, 1.571 ]
}