When embedding a card inside another one (e.g. a custom card inside an entities
card, common with customized cards, e.g. mini-climate - see artem-sedykh/mini-climate-card#122), the themes seems to generate a box-shadow.
Commenting out the ha-card-box-shadow
variable seems to do the trick, but that's obviously a hack for someone who doesn't understand much about themes, like me.
From what I can see in the inspector, it seems such embedded cards are materialized as ha-card.--group
(among other classes), as opposed to the root ones which are just ha-card
with no classes.