Well, whatever happens, I hope the guest of honor wouldn't run out at any point to dig up her own grave - or she wouldn't be kidnapped by a ghost and thrown into an open grave - or she wouldn't have her face turn into a skull while wearing her veil and opening gifts.
But seriously, this is a wild idea - but I agree it's hard to conceive a theme...
Well, she could adapt the English Tea Party theme that a lot of women seem to use. And to make it truly DS, guests could be given bows to put in their hair.
Though I suspect that would probably be way too vanilla for what she has in mind...
Perhaps they could forgo an overarching theme and simply go with DS style activities and or foods? Perhaps they could play some sort of DS themed game(s)? (Though I suppose they couldn't be too specific to DS so that non-fans would have a chance to win.) Perhaps DS themed pastries? (Cookies or cupcakes or whatever decorated with bats, ghosts, etc.) Of course, there would be absolutely no garlic in the cheese dip. And there's always sardine canopes. (Don't knock them 'til you've tried them.
) But I hope that whatever your niece ends up doing, you'll be able to share a photo or two.