How to change the Backcolor from a Media-Player?

Hello,

How can I change the background of a media player?

```python
type: custom:bubble-card
card_type: media-player
entity: media_player.wave_iv_3
styles: |-
    .bubble-button-background {....
``` 

"bubble-button-background" does not work.

Thanks for helpful tips.

Gunter