Dear Hans
@Hans-Pelle
This is possible with some custom coding, but not with the current built-in Customizer options.
It would require two images to be loaded with each thumbnail.
You may wish to seek the aid of a developer if needed.
Also the documentation page will help you further:
https://laytheme.com/documentation.html#custom-css-styling
Best wishes and have a wonderful day Hans š¼
Sincerely
Richard