well graphics is a personal preference, so not necessarily closed minded, but as far as changing skins, as far as I know you would have to purchase it. The way they do it on the pc is, when you purchase it you sign up for a user name to sign in with, and then after you purchased it, you log into their website and on the page after you log in you will see an upload button for the custom skins.
BUT i am not 100% sure if this is how the mobile device version works
also on the pc, single player is free, multiplayer is unlocked with the purchase.
For the android version I think the demo is just a creative mode, which is unlimited blocks (which i read is limited to 32 blocks to choose from) to build with. The purchased version has survival mode as well as creative. The creative would be similar, but with a few additions, she could save the game when she wants to quit (which you cant do with the free) More blocks to build with, and Monsters that won't attack unless attacked first.
Survival mode is just that.. survival, you have a health bar, and food level. You have to make things to survive, you have to first chop down trees with your hands to make a crafting table then make an ax with the wood you got to make it easier to chop down trees. You'll want to make a house with a bed in it so you have shelter in the night, because thats when the monsters are out (spiders, zombies, skeletons, and creepers) they will attack you at night.
It is a nice game the encourages creativity! hope this is of help to you