the skins are only used on the wood firmware for this card, that is when you use the blue card to play older DS/DSi games, when using the cart for 3ds games it uses the actual console interface so no skins are needed, that being said if you require a skin for the wood firmware i would suggest taking one of the pre built ones and customising it to your requirements.

You can find some here for various r4 cards http://r4town.com/pages/R4-Skin.html