Look at them all and you'll see each one has a common part of the name. Take the desert sky for instance. It's parts are desertup, desertdn, desertft, desertbk, desertlf and desertrt. Each one has the word desert in it. So the sky name is desert. Watch out for textures with underscores at the end though, like cs's snowlake_ sky. Originally I entered snowlake and it didn't work, 'cos I ignored the underscore.