Absolutely no connection to the theme that I can see, not even a shoe-horned in - you died or all the characters are undead.
Nice animations. I didn't understand what the different colours meant, if anything? Game seemed fairly easy, does it ever end? I rarely even got hit. I didn't even upgrade my cards, upgrading 5 cards at once (but one at a time) was tedious and a bit overwhelming.
Nice idea, decent execution but could use some balancing