Change icon for "toggle basemap" button in Basic Viewer

[TórshavnarKommuna]

Hi.

I would like to set an icon for "toggle basemap" button in Basic viewer. If the button is added to the map, the icon for the "secondary" map is correct, but the icon for the default basemap is not really representative of the basemap itself. Is there anyway how to change the icon?

Image below shows the issue:

Thank you.

Martin