I want to check if I'm missing something or if the "Highlights Panel" component that allows custom actions to be added just does not (by design) show custom action icons. I assume the latter; please confirm or let me know what I'm missing here.
(Would be good if it did - a long row of text-only custom buttons looks pretty dull.)
Here is my custom action with a red icon referenced via a static resource:
and here is the action presented without the icon: