*edited*
All you should have to do is edit the moon texture here:
FeralHeart > media > textures > moon.png
It uses black for its background so if you want to make your own moon from scratch you could make it as different file with transparency and then copy only the visible stuff onto just a filled black form of the actual moon texture. =o
I recommend making a backup or two just in case something goes wrong. =)
I use gimp 2.6 for stuff like this.