Hello,
Sorry, too far for my small knowledge.
Not sure how to remove it but you can work with margin and negative number px on card to reduce spacing and put background to none.
This will not remove background but will reduce spacing.
You can try to ask in the thread for card mod.
https://community.home-assistant.io/t/card-mod-add-css-styles-to-any-lovelace-card/120744