Are you looking to change the background of the whole card. I don’t think that this is supported (and would be use case for that) since there is no card global entity that could be based on.
If you are attempting to change the image for one of the entities shown in the card, then make sure that the values in the config (e.g. Day and Night in your example) match the actual state of the entity (check via the HA dev tools).
yes, I’m trying to change the background of the whole card, just some experiments to better understand how works the different cards.
thanks for your support