As a poke and guess admin I was a bit worried reading this, but when I checked my custom theme, I saw only a very simple script:
<script type="text/discourse-plugin" version="0.8.11">
// icon substitutions for share buttons
api.replaceIcon('d-post-share', 'up-right-from-square');
api.replaceIcon('d-topic-share', 'up-right-from-square');
</script>
So it was just a copy of whats between the script tags, hop to the JS tab, and there was a place to slap it in. That was easy!
import { apiInitializer } from "discourse/lib/api";
export default apiInitializer((api) => {
// icon substitutions for share buttons
api.replaceIcon('d-post-share', 'up-right-from-square');
api.replaceIcon('d-topic-share', 'up-right-from-square');
});