I would say they are neither white with black stripes nor black with white stripes. Both colors work together equally. I've never seen a Zebra that is either black or white. Additionally, an individual Zebra may have more or else shades of black and white than another. So, it is hard to say if they are predominantly black or predominantly white.
Apparently their skin is black but the hair is multi-coloured. Newborns can be born with brown stripes which then turn black later, so that leans me towards white with black stripes :)