Attaching Theme Library to Custom Module I have a subtheme called bootstrap_subtheme. It has a library called global-styling. I created a custom module called telephone_alter. In telephone_alter.module, I have the following:
function telephone_alter_form_user_form_alter(&amp;$form){
  $form['#attached']['library'][] = 'bootstrap_subtheme/global_styling';
}

it does not load the bootstrap_subtheme library stylesheet. I do dump($form) and get the following:
  &quot;#attached&quot; =&gt; array:1 [▼
    &quot;placeholders&quot; =&gt; array:1 [▼
      &quot;form_action_p_pvdeGsVG5zNF_XLGPTvYSKCf43t8qZYSwcfZl2uzM&quot; =&gt; array:1 [▼
        &quot;#lazy_builder&quot; =&gt; array:2 [▶]
      ]
    ]
  ]

seems like the attachment isn't even grabbing it. Yes I am using the right form alter according to the api: hook_form_FORM_ID_alter() and my ID is user_form . It does fire off, if I attach css from within my module's folder, but not my subtheme. Any help would be great! Thank you.
One more note:
my root dir is as follows:
modules
   -telephone_alter
theme:
   contrib:
      -bootstrap
   custom:
      -bootstrap_subtheme

Here is bootstrap_subtheme.libraries.yml:
global-styling:
  js:
    js/popper.min.js: {}
    js/bootstrap.min.js: {}
    js/global.js: {}
  css:
    component:
      css/style.css: {}
  dependencies:
    - core/jquery
    - core/drupal
Based on that structure, you're referencing the library incorrectly.
It should be $form['#attached']['library'][] = 'bootstrap_subtheme/bootstrap_subtheme.global-styling';
As noted in a comment below, the duplicate use of the theme name may not be necessary, and $form['#attached']['library'][] = 'bootstrap_subtheme/global-styling'; should work according to the docs. (Note the use of a dash, matching the library name versus an underscore in your code.)