As of now, it seems we cannot apply a custom style for a combo popup menu using CSS.
The only way to do it, is to overwrite .combo-box-popup .list-view {} and .combo-box-popup .list-view .list-cell {} but this sets the style globally to all ComboBox.

If I set a custom style "my-combo-style" to a ComboBox, there should be a way to set a custom style to this combo's popup too.
This needs to be addressed or more documentation and examples need to be posted to explain how to achieve setting a custom style to a combo's popup.