Lovelace: Button card

today I found myself struggling using a card variables inside a cards background css gradient. I did mange to use it in the end, but it took an intermediate variable I had to declare on the element itself.

I had hoped to do something like:

- background: radial-gradient(variables.severity_color ,skyblue)

but that wont work, nor some of the version I tried with ${variables.severity_color} inside a html format.

Feels a bit clunky to have a global variables set, and not be able to use it directly in one of the options, but need to ‘return’ it first to a new variable.

Please have a look what I did wrong/how this could be fixed If at all?
thanks!

type: custom:button-card
template: button_summary
entity: sensor.schimmel_sensor
name: Schimmel
variables:
  active: >
    [[[ return states['binary_sensor.schimmel_alert'].state === 'on'; ]]]
  severity_color: >
    [[[ var mold = entity.state;
        if (mold > 80) return 'red';
        if (mold > 70) return 'crimson';
        if (mold > 65) return 'orange';
        return 'green'; ]]]
hold_action:
  action: navigate
  navigation_path: /ui-data/klimaat
state:
  - operator: template
    value: >
        [[[ return variables.active;]]]
    spin: true
styles:
  card:
    - color: var(--primary-color)
    - --card-color: >
        [[[ return variables.severity_color; ]]]
## wanted to use the variables.severity_color inside the background here
    - background: radial-gradient(var(--card-color) ,skyblue)
  icon:
    - color: >
        [[[ return variables.severity_color; ]]]