This may never happen.
Suggest to ask a question in the main card-mod thread.
Besides, the 1st post of that thread contains a link (in the end) which leads to a list of examples, find there examples for bar-card, might be useful.
In general, card-mod should be used as
type: custom:bar-card
...
card_mod:
style: ....
Not that it is defined on a card’s level, not per-entity; and also note that missing “card_mod” keyword.