Alright. If you want to make skyboxes, I recommend making a 2:1 canvas in Krita and turning on the features that tiles the canvas endlessly. It helps making seamless textures. And you can get a sky-esque feel with just a vertical gradient and undefined potentially blurred brushstrokes. Also keep in mind that the closer you get to the middle of the height, the more that part will be stretched vertically in-game, so that's where you want to put the highest density of details. Finally, when you import the texture in unity, it has to be a cubemap.
Hope this can help, have a good day.