I'm not sure how the M3DS real works but I do have a friend that owns one and was able to easily manipulate the skin.
If you have a skins folder on your card, check the file content. If there's images, just manipulate them with an image program like Fireworks, Paint Shop Pro or Photoshop. I'm pretty sure they all work that way.
There should also be a file called skins.xml or skins.ini depending on the card you have that will enable you to modify the font colours and background colours used with the skin.
I'm just guessing this but I'm pretty sure this is how it works.
30/05/2008
clayton, melbourne, victoria