[mini-graph-card] How can I set the background of the graph

Hello,

I want the background of my mini-graph-card to be transparent
image

But I couldn’t figure out how.

I tried with styles backgroud-color, with ha-card {} and so on, but I don’t get it…

Can someone please point me into the right direction?

type: custom:button-card
aspect_ratio: 10/3
custom_fields:
  value:
    card:
      type: custom:button-card
      entity: sensor.wohnzimmer_klima_temperature
      show_icon: false
      show_state: true
      show_name: true
      name: "Indoor Temperature :"
      styles:
        name:
          - padding-left: 70px
          - justify-self: start
          - font-size: 0.9em
          - font-weight: bold
        state:
          - padding-left: 100px
          - justify-self: start
          - font-size: 0.7em
        card:
          - background-color: transparent
          - width: 500px
          - border-radius: 0%
          - z-index: 1
      tap_action:
        action: none
      hold_action:
        action: none
  graph:
    card:
      type: custom:mini-graph-card
      entities:
        - entity: sensor.wohnzimmer_klima_temperature
      color_thresholds:
        - value: -20
          color: "#0000FF"
        - value: -10
          color: "#00BFFF"
        - value: 0
          color: "#ADD8E6"
        - value: 10
          color: "#00FF00"
        - value: 20
          color: "#80FF00"
        - value: 25
          color: "#FF8000"
        - value: 30
          color: "#FF0000"    
      hours_to_show: 24
      hour24: true
      points_per_hour: 2
      animate: true
      update_interval: 30
      aggregate_func: avg
      line_width: 2
      bar_spacing: 5
      height: 100
      group: true
      show:
        labels: false
        fill: fade
        state: false
        name: false
        icon: false
styles:
  card:
    - background-color: var(--button-card-background-color)
  custom_fields:
    graph:
      - position: absolute
      - bottom: 0px
      - left: 0px
      - width: 100%
      - height: 70%
      - padding-bottom: 0
    card_mod:  
tap_action:
  action: none
hold_action:
  action: none