Like Autumn explained, it's only a matter of drawing over the moon what you want for the mod (or using an image such as the earth). However because of the way the codes are for the moon texture in game, you want to make sure that the background of your new moon texture is black like what you see in Autumn's examples, because unless you change the codes, it could come out a bit wonky.
As for the sun I believe it's the same concept as the moon. Find the texture C:>FeralHeart>media>textures and open it in GIMP, then simply draw over the sun what you want (or again use an image if you want), and also make sure that the background is black and save as sun.png in your textures folder. Should work then.
I wouldn't recommend changing the sizes of the moon or sun images, or else it could look a bit strange, but I won't discourage you from trying something new! Maybe changing the sizes to bigger dimensions will give you a more HD sun or moon? Not recommended, but try it out if you please.