Skybox can be a little bit more than just a pretty picture to cover the infinite void around our level; its properties can play a huge role in how our level is going to be lit:
Go to Window | Lighting.
In the Scene tab, under Environment Lighting, find the Skybox parameter.
Default-Skybox is a special material that exists in every Unity project and assigned as a default skybox. You can't edit, delete, or even find this material anywhere in the editor. However, you can replace it with your own custom skybox material.
To add a custom skybox, simply create a material and change its Shader type to Skybox | Procedural.
Now we have a new customizable Skybox that we can tweak and replace the default one with. Try to manipulate the sliders to set an interesting mood for your level, such as sunset or nighttime.