Problem/Motivation

I’m interested in altering/overriding the little bit of HTML markup that’s output by core, in my case Drupal.theme.mediaEmbedEditButton (from core/modules/media/js/media_embed_ckeditor.theme.js), but without hacking core, of course. It seems I could do this in a custom theme by adding:

libraries-extend:
  media/media_embed_ckeditor_theme:
    - my_theme/my_library

(plus the info for my_library in my_theme.libraries.yml, and the JS file in my theme with the function that would return the new markup).

However, since this shows up in places like /node/add/page that only uses the admin theme (we’re using Seven), such additions to my_theme wouldn’t apply. While it seems like I could make a bare-bones my_admin_theme, pulling in Seven as base, I’d like to keep this override near other related customizations in a custom module (I also ran into other cans of worms when I tried this as a proof of concept).

What (else) I’ve tried

I tried putting in libraries-extend: from above in my_module.info.yml (plus the addition to my_module.libraries.yml and the new JS, of course), but this didn’t seem to work (kinda not surprising, since the docs I’m finding only seem to reference libraries-extend in the same breath as themes). I also tried using hook_field_widget_form_alter() in my_module.module to attach my override library via $element['#attached']['library'][] but didn’t have luck there either.

Am I stuck with with making a custom admin theme if I want to use libraries-extend? (I’m guessing this is also a problem for libraries-override.) Or am I missing some hook or something I could instead add to my custom module to accomplish overriding the output made by Drupal.theme.mediaEmbedEditButton?