Home Assistant screensaver has stopped working

Third party integrations
1

I have written my own screensaver within Home Assistant using an automation, a motion sensor and the browser_mod popup. Vital to my routine is the popup Style: which is used to colour the background of the popup black.

Here is my YAML code:

alias: Dashboard PC - Hide Screen
description: 
trigger:
  - type: no_motion
    platform: device
    device_id: 590fa3e9b70acf994f210ed7c410c340
    entity_id: binary_sensor.hall_motion_sensor_ias_zone
    domain: binary_sensor
    for:
      hours: 0
      minutes: 0
      seconds: 0
condition: []
action:
  - service: browser_mod.popup
    data:
      dismissable: true
      autoclose: true
      size: fullscreen
      style: "--popup-background-color: black;"
    target:
      device_id:
        - e357dd4e623671a74dd9c232c5139780
mode: single

However recent changes to browser_mod mean that it no longer changes to a black screen but rather goes white. We are told by xZetsubou on GitHub:

“this is not a bug the --popup-background-color variable has been removed in 2.3.0 70a5cbc and now its use the same background color as HA popups like more-info you can change the background either by card_mod or in your theme yaml
add card-background-color: transparent this will be applied in all HA popup include BM popups.”

Unfortunately I have not found this the clearest explanation and have been unable to restore the functionality I had before. Can anyone help?