Like sassyfras said, dog food doesn't have the taurine that cats must have in their diet. As such, dog food isn't harmful for cats, but it is insufficient as their only or main food. Cat foods usually have more protein than dog foods, too, and since cats are obligate carnivores, a diet low in protein is not appropriate for them (not that it's appropriate for dogs, but won't get into that here). So, as a very short term thing, I don't think it's going to harm your cat, but I would recommend getting some cat food very soon.