Background color of a single tab

I found how to have a background picture displayed in a dashboards for a single tab.

  - title: test
    background: >-
      center / cover repeat
      url("https://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Firefox_logo%2C_2019.svg/300px-Firefox_logo%2C_2019.svg.png")
      fixed

But how to set the background color of a single tab?