These gradients are simple to do. If you work with Gimp instead of Paint.net you do the same thing. Take two colors and then gradient tool.
But how about gradients that are supposed to look like something. Like a star sky? Well, for that you don't want to use the small gradient picture but the big one. The big one is the same size as the ks play window. Just take a picture and paste it in the gradient so it covers everything or work free.