Suggestion:
If you need to style smth by card-mod - do not create a separate thread. Go to the huge card-mod thread and ask there.
Also, there is a dedicated huge mushroom thread - may be it is better to ask there.
Reasons: keep solutions in 1 place, less time for searching, more people will see your question, get answers faster.
For many other cards I can give an advise “how to make it transparent”, but not for mushrooms, not using them.
Just a guess - try this:
type: mushroom….
…
card_mod:
style: |
ha-card {
--ha-card-background: ….
}
Also - always post a code as a code, not as a picture.
There is a special formatting here (triple “`”).