The values

SkyOverride is what you hand to Set. Every property is nullable: a property you leave null is not changed by your mod. SkyState is what you get from Current. It has the same names, and every value is read only.

SkyOverride the values your mod wants to change

WorldSky.Set takes one of these, with only the values you want to change filled in.

public sealed class SkyOverride
{
    public bool? FogEnabled { get; set; }
    public FogMode? FogMode { get; set; }
    public Color? FogColor { get; set; }
    public float? FogDensity { get; set; }
    public float? FogStart { get; set; }
    public float? FogEnd { get; set; }
    public Color? AmbientColor { get; set; }
    public float? AmbientIntensity { get; set; }
    public float? SkyboxExposure { get; set; }
    public Color? SunColor { get; set; }
    public float? SunIntensity { get; set; }
    public float? StarsIntensity { get; set; }
    public float? CloudDensity { get; set; }
}

Property	Type	What it means
FogEnabled	bool?	true turns fog on. false turns it off.
FogMode	FogMode?	How the fog thickens with distance (Unity’s FogMode).
FogColor	Color?	The colour of the fog.
FogDensity	float?	How thick the fog is. 0 or more.
FogStart	float?	The distance where linear fog starts.
FogEnd	float?	The distance where linear fog is full.
AmbientColor	Color?	The colour of the light that fills shadows.
AmbientIntensity	float?	How strong that light is. 0 or more.
SkyboxExposure	float?	How bright the sky is. 0 or more.
SunColor	Color?	The colour of the sunlight.
SunIntensity	float?	How strong the sunlight is. 0 or more.
StarsIntensity	float?	How bright the stars are. 0 or more.
CloudDensity	float?	How thick the clouds are, from 0 to 1.

SkyState what the sky looks like now

WorldSky.Current gives you one of these, which is the sky as it looks at that moment.

public sealed class SkyState
{
    public bool FogEnabled { get; }
    public FogMode FogMode { get; }
    public Color FogColor { get; }
    public float FogDensity { get; }
    public float FogStart { get; }
    public float FogEnd { get; }
    public Color AmbientColor { get; }
    public float AmbientIntensity { get; }
    public float? SkyboxExposure { get; }
    public Color? SunColor { get; }
    public float? SunIntensity { get; }
    public float? StarsIntensity { get; }
    public float? CloudDensity { get; }
}

Property	Type	What it means
FogEnabled	bool	true when fog is on.
FogMode	FogMode	The fog type.
FogColor	Color	The fog colour.
FogDensity	float	The fog thickness.
FogStart	float	Where linear fog starts.
FogEnd	float	Where linear fog is full.
AmbientColor	Color	The colour of the light that fills shadows.
AmbientIntensity	float	How strong that light is.
SkyboxExposure	float?	null when the scene has no sky that carries it.
SunColor	Color?	null when the scene has no sun light.
SunIntensity	float?	null when the scene has no sun light.
StarsIntensity	float?	null when the scene has no sky that carries it.
CloudDensity	float?	null when the scene has no sky that carries it.

Things to know

• The fog and ambient values in SkyState always have a value. The others are null when the scene does not contain the object that holds them. A scene can use a different mix of sky systems than another, so a value you can read in one scene may be null in the next. (the game’s scenes use SunDirection, the Azure sky controllers, or both. A property whose object is missing is skipped, and Current reports it as null.)
• AmbientColor sets every kind of ambient light to the same colour. (flat, sky, equator and ground ambient colours all get the value.)
• SkyboxExposure changes the sky’s brightness whichever way the scene draws its sky. It sets the _Exposure property of the current skybox material when it has one, and the exposure of the Azure sky controller when one is in the scene.
• SunColor and SunIntensity change every sun light the sky systems drive. In the game the sun and the moon share one light, so this changes the moonlight too.

Limits

• The Azure fog is not the same as the fog you change here. The game draws its own fog as a screen effect from the sky’s shader values, and these properties change Unity’s built-in fog only. So FogEnabled and the other fog values may look different from the Azure fog you see. (Azure fog is driven by globalFogDistance and other shader values, not by RenderSettings.fog.)