As the skybox cannot be transitioned, it is not part of the MainDayNight.cs script. However, there is a C# function in MainRender.cs that can be called in the events panel to change the skybox. As for the background color, I think I'll be able to add a transition in a future update.
SetSkyBox
it uses a string with the exact skybox name in your resources, example = "sb_obj_sky_Night"