added islandora.libraries.yml

css/islandora.theme.css

@@ -0,0 +1,3 @@
+.container .islandora-media-items {
+  margin: 0;
+}

islandora.libraries.yml

@@ -0,0 +1,5 @@
+drupal.islandora.theme_css:
+  version: VERSION
+  css:
+    theme:
+      css/islandora.theme.css: {}

islandora.module

@@ -357,8 +357,8 @@ function islandora_form_alter(&$form, FormStateInterface $form_state, $form_id)
           '#theme' => 'item_list',
           '#type' => 'ul',
           '#items' => $media_list,
-          '#attributes' => ['class' => 'mylist'],
-          '#wrapper_attributes' => ['class' => 'container'],
+          '#attributes' => ['class' => ['islandora-media-items']],
+          '#wrapper_attributes' => ['class' => ['container']],
           '#attached' => [
             'library' => [
               'islandora/drupal.islandora.theme_css',