Help customizing a built-in card

Tags: #<Tag:0x00007fc4183531b0>

I would like to make some minor changes to the built-in thermostat card. Members of my family find the display of current temp vs. set temp confusing. I’d like to tweak the layout and text. I need help understanding the right way to do this, if there is one.

As far as I can tell the source for the card is not available in my installation to be edited, so I tried saving it as a resource under www, linking to it from my configuration, and then using it as if I had written it, following the directions in the developer docs. Unfortunately this does not work, I assume because of the extensive importing of resources at relative locations.

Is there a reasonable way to do this, take a built-in card, apply some mild changes and use it as a custom card? I’m pretty sure that the changes I’d like to make go beyond what is possible with card mod.