Texture = 
TexturePack = 
Weather = -1
EdgeLevel = 32
SidesOffset = -1
CloudsHeight = -1
MaxFog = -1
clouds-speed = -1
weather-speed = -1
weather-fade = -1
skybox-hor-speed = -1
skybox-ver-speed = -1
HorizonBlock = 255
EdgeBlock = 255
ExpFog = -1
CloudColor = 
FogColor = 
SkyColor =
ShadowColor = 
LightColor = 
SkyboxColor =

What all of these do? Can you explain, especially can i change skybox texture to my own?

To put it simply; these change how the level looks.

Textures and Texturepack control the textures of the level (and there needs to be a skybox texture in the texture pack in order to have a skybox.)

Weather and it's speed and fade variables changes the weather,

SidesOffset is obvious, it sets the side blocks (the blocks beneath the edge/horizon) offset either up or down depending on the value,

MaxFog and ExpFog is the fog settings with maxfog changing how far away or close the fog is, and expfog.. turns on the fog on the edges,

The skybox has some horizontal and vertical speeds,

HorizonBlock and EdgeBlock allows you to change the edge and horizon blocks, by default; it's water (8) and bedrock (7),

Then; you can set colors of the Clouds, Fog, Sky, Shadows (the circle or square below your character or under bots, and the shadow of blocks), Lights and the Skybox color.

 

You can also change these variables with /env <variable value> in-game. Hope this helps.

thanks