There could also be a circulation problem that the kitten was born with, which would also cause the affected paw to be cold to the touch. Yes, it could be frostbite, too-IF the cat was in that office building with no heat. Also, you might be right-the paws might be burned by some sick freak. Does the paw pad look infected-like oozing pus?
I think that was great of you to take the kitten in, especially around X-mas!!
Please, give us an update-I'd love to hear how things go!
Just as a quick thought you aren't in an area that's particularly cold right now or has ice on the ground? even cold concrete can cause the equivalent of frost burns to the bottom of a cat's feet.
Otherwise dark pigmentation spots on the pads of a cat's paws isn't an unusual occurance. If there appears to be no damage, don't fret, all is well!