Here is their own website. I would guess that it is one background, not four, so ask the seller.
I would go for holographic skin, especially if they allowed to bring my own images instead of choosing one of their designs, but the holographic add-ons I have seen were made for iPhone (not fitting on N900) and seemed to be quite fragile.
In the future, though...
Best wishes.
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Per aspera ad astra...