Always fancied tinting the rear windows on my panda. Mainly to try and keep the sun off the dog when he's in the car, but I think the white (my) car and black tints would go well.
100HP has darker rear windows. I once had a black Punto Mk2 with very dark rear windows. Some said it was fab. Others said it looked like a van.
Dark windows wont be much help for the dog. Better to get sun blinds and drive with windows open. Fronts open 2" cause better air circulation that backs and they don't thump with air turbulence.