I have an IKEA Fyrtur blind, with the standard “blind” icons associated with it.
As the Custom Brand Icon set has the icons for both the open and closed fyrtur blind (via phu:ikea-blind-open and phu:ikea-blind-closed), I want to use them both.
You cannot use templates in standard “customize”.
If this “cover” entity is provided by integration (i.e. not a template cover) - your possible solutions are:
Google for Custom UI - it allows (at least allowed 2 years ago, have not tested it since that time) to use JS-templates in “customize”. An icon is changed in ALL cards & more-info window.
Replace an icon on a particular card by card-mod (ask details in a dedicated thread).
For a template cover - you can define an icon in a template.