diff -u b/core/themes/seven/seven.theme b/core/themes/seven/seven.theme
--- b/core/themes/seven/seven.theme
+++ b/core/themes/seven/seven.theme
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\media\MediaForm;
 
 /**
  * Implements hook_preprocess_HOOK() for HTML document templates.
@@ -167,9 +168,10 @@
  * Implements hook_form_BASE_FORM_ID_alter() for \Drupal\media\MediaForm.
  */
 function seven_form_media_form_alter(&$form, FormStateInterface $form_state) {
+  $media_is_enabled = \Drupal::moduleHandler()->moduleExists('media');
   // Only attach CSS from core if this form comes from Media core, and not from
   // the contrib Media Entity 1.x branch.
-  if (\Drupal::moduleHandler()->moduleExists('media')) {
+  if ($media_is_enabled && $form_state->getFormObject() instanceof MediaForm) {
     // @todo Revisit after https://www.drupal.org/node/2892304 is in. It
     // introduces a footer region to these forms which will allow for us to
     // display a top border over the published checkbox by defining a