Issue #2667242 by Anul, felribeiro, eojthebrave, pranavbabbar: Use consistent terminology when referencing administration themes in the UI (not "admin theme")

core/lib/Drupal/Core/Extension/ThemeInstaller.php

@@ -223,7 +223,7 @@ class ThemeInstaller implements ThemeInstallerInterface {
         throw new \InvalidArgumentException("The current default theme $key cannot be uninstalled.");
       }
       if ($key === $theme_config->get('admin')) {
-        throw new \InvalidArgumentException("The current admin theme $key cannot be uninstalled.");
+        throw new \InvalidArgumentException("The current administration theme $key cannot be uninstalled.");
       }
       // Base themes cannot be uninstalled if sub themes are installed, and if
       // they are not uninstalled at the same time.

core/modules/node/config/schema/node.schema.yml

@@ -6,7 +6,7 @@ node.settings:
   mapping:
     use_admin_theme:
       type: boolean
-      label: 'Use admin theme when editing or creating content'
+      label: 'Use administration theme when editing or creating content'
 
 node.type.*:
   type: config_entity

core/modules/system/src/Controller/SystemController.php

@@ -307,7 +307,7 @@ class SystemController extends ControllerBase {
         $theme->notes[] = $this->t('default theme');
       }
       if ($theme->is_admin) {
-        $theme->notes[] = $this->t('admin theme');
+        $theme->notes[] = $this->t('administration theme');
       }
 
       // Sort installed and uninstalled themes into their own groups.

core/themes/seven/seven.info.yml

@@ -4,7 +4,7 @@
 # As the UI of Drupal improves between minor versions, the mark up and assets
 # in the Seven theme will change. The Seven theme is not backwards
 # compatible. If you wish to modify the output or assets of Seven you can:
-# 1. Copy the whole of Seven and rename it as your own admin theme. You
+# 1. Copy the whole of Seven and rename it as your own administration theme. You
 # will need to manually manage your own updates if you want to stay up to
 # date with Seven's bug fixes and feature support.
 #