Your comments

And my reply :

Indeed, I preferred to switch the icon to color (not tested in gray but I imagine that it works) and it appears well in Dark Mode.
It seems to me that the icon is made lighter in Dark Mode while the expected result to be consistent with the Apple system is a white icon in Dark Mode (black in light mode).

It would still be interesting to be able to decide on the icon for each mode and to be able to use a Retina icon.
But I understand that there is surely more urgent to do ...

Support response :

we believe this to be expected behavior when using dark mode and a dark icon. I see that your image is a PNG with no background and only using a dark color so its going to blend into the system. You need to use a lighter color or shade in your icon or put a background on the image and that should allow you to see the icon.

thank you, the ticket is open: 14012681

Thank you for your reply.
The Mac is under 10.15.7.
Are you talking about a personalized icon, not the original?

Yes, since version 5.6, after a first connection, consent dialog not displayed until the machine is rebooted or the service would be nice to have an option to force the display of consent dialog each time.