You change the display options like sky and ground color as well as many others by adjusting the style. There are various styles that ship with Sketchup, and you can adjust to make your own, it is personal preference. Different styles are useful for different activities.
There is a lot of information on adjusting and saving styles on the forum if you search.
Open a new file, set up styles,scenes,etc the way you would like them. Then File> save as template. Choose to make that template default if you want it to open every time.
On the part about choosing your own template to be the default, in 2019 and 2020 you can do that by looking at the Files section of the Welcome to SketchUp screen. In each template thumbnail is a white circle. Point at that and it becomes a heart icon. Click on the white circle of the template you want to be default.