Thanks so much for the pointer - used Shortcuts as per the linked article and it works.
Default and “improved” icon shown in screenshot.
Yep, the new icon is badly scaled, and I had to zoom all the way to avoid Shortcuts giving it green borders … but it’s so much less harsh looking than the default white box?
Looking forward to a “proper” dark mode icon, this will do me for now.
Used icons from Big Sur icon - after that screenshot, I switched to the icon from that link with a lighter background, and suspect all this really needed for me was not having harsh rgb(255,255,255) white surrounding the icon.