diff --git a/src/Form/StylesDeleteForm.php b/src/Form/StylesDeleteForm.php
index 58c2847..73a18cf 100644
--- a/src/Form/StylesDeleteForm.php
+++ b/src/Form/StylesDeleteForm.php
@@ -38,14 +38,12 @@ class StylesDeleteForm extends EntityConfirmFormBase {
   public function submitForm(array &$form, FormStateInterface $form_state) {
     $this->entity->delete();
 
-    drupal_set_message(
-      $this->t('content @type: deleted @label.',
-        [
-          '@type' => $this->entity->bundle(),
-          '@label' => $this->entity->label(),
-        ]
-      )
-    );
+    $this->messenger()->addStatus($this->t('content @type: deleted @label.',
+      [
+        '@type' => $this->entity->bundle(),
+        '@label' => $this->entity->label(),
+      ]
+    ));
 
     $form_state->setRedirectUrl($this->getCancelUrl());
   }
diff --git a/src/Form/StylesForm.php b/src/Form/StylesForm.php
index 4ed29c0..e29d177 100644
--- a/src/Form/StylesForm.php
+++ b/src/Form/StylesForm.php
@@ -125,13 +125,13 @@ class StylesForm extends EntityForm {
 
     switch ($status) {
       case SAVED_NEW:
-        drupal_set_message($this->t('Created the %label Styles.', [
+        $this->messenger()->addStatus($this->t('Created the %label Styles.', [
           '%label' => $styles->label(),
         ]));
         break;
 
       default:
-        drupal_set_message($this->t('Saved the %label Styles.', [
+        $this->messenger()->addStatus($this->t('Saved the %label Styles.', [
           '%label' => $styles->label(),
         ]));
     }
diff --git a/src/Plugin/Field/FieldType/StyleEntityReferenceItem.php b/src/Plugin/Field/FieldType/StyleEntityReferenceItem.php
index 6585809..3116771 100644
--- a/src/Plugin/Field/FieldType/StyleEntityReferenceItem.php
+++ b/src/Plugin/Field/FieldType/StyleEntityReferenceItem.php
@@ -63,11 +63,11 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
    */
   public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
     $settings = $field_definition->getSettings();
-    $target_type_info = \Drupal::entityManager()->getDefinition($settings['target_type']);
+    $target_type_info = \Drupal::service('entity_type.manager')->getDefinition($settings['target_type']);
 
     $target_id_data_type = 'string';
     if ($target_type_info->entityClassImplements(FieldableEntityInterface::class)) {
-      $id_definition = \Drupal::entityManager()->getBaseFieldDefinitions($settings['target_type'])[$target_type_info->getKey('id')];
+      $id_definition = \Drupal::service('entity_field.manager')->getBaseFieldDefinitions($settings['target_type'])[$target_type_info->getKey('id')];
       if ($id_definition->getType() === 'integer') {
         $target_id_data_type = 'integer';
       }
@@ -112,7 +112,7 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
    */
   public static function schema(FieldStorageDefinitionInterface $field_definition) {
     $target_type = $field_definition->getSetting('target_type');
-    $target_type_info = \Drupal::entityManager()->getDefinition($target_type);
+    $target_type_info = \Drupal::service('entity_type.manager')->getDefinition($target_type);
     $properties = static::propertyDefinitions($field_definition)['target_id'];
     if ($target_type_info->entityClassImplements(FieldableEntityInterface::class) && $properties->getDataType() === 'integer') {
       $columns = [
@@ -285,7 +285,7 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
       'entity' => NULL,
     ];
 
-    $entity_type = \Drupal::entityManager()->getDefinition($options['target_type']);
+    $entity_type = \Drupal::service('entity_type.manager')->getDefinition($options['target_type']);
     $options['handler_settings']['sort'] = [
       'field' => $entity_type->getKey('id'),
       'direction' => 'DESC',
@@ -464,14 +464,16 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
    */
   public static function calculateDependencies(FieldDefinitionInterface $field_definition) {
     $dependencies = parent::calculateDependencies($field_definition);
-    $manager = \Drupal::entityManager();
+    // TODO: Drupal Rector Notice: Please delete the following comment after you've made any necessary changes.
+    // We are assuming that we want to use the `entity_type.manager` service since no method was called here directly. Please confirm this is the case. See https://www.drupal.org/node/2549139 for more information.
+    $manager = \Drupal::service('entity_type.manager');
     $target_entity_type = $manager->getDefinition($field_definition->getFieldStorageDefinition()->getSetting('target_type'));
 
     // Depend on default values entity types configurations.
     if ($default_value = $field_definition->getDefaultValueLiteral()) {
       foreach ($default_value as $value) {
         if (is_array($value) && isset($value['target_uuid'])) {
-          $entity = \Drupal::entityManager()->loadEntityByUuid($target_entity_type->id(), $value['target_uuid']);
+          $entity = \Drupal::service('entity.repository')->loadEntityByUuid($target_entity_type->id(), $value['target_uuid']);
           // If the entity does not exist do not create the dependency.
           // @see \Drupal\Core\Field\EntityReferenceFieldItemList::processDefaultValue()
           if ($entity) {
@@ -503,7 +505,7 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
    */
   public static function calculateStorageDependencies(FieldStorageDefinitionInterface $field_definition) {
     $dependencies = parent::calculateStorageDependencies($field_definition);
-    $target_entity_type = \Drupal::entityManager()->getDefinition($field_definition->getSetting('target_type'));
+    $target_entity_type = \Drupal::service('entity_type.manager')->getDefinition($field_definition->getSetting('target_type'));
     $dependencies['module'][] = $target_entity_type->getProvider();
     return $dependencies;
   }
@@ -513,7 +515,9 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
    */
   public static function onDependencyRemoval(FieldDefinitionInterface $field_definition, array $dependencies) {
     $changed = parent::onDependencyRemoval($field_definition, $dependencies);
-    $entity_manager = \Drupal::entityManager();
+    // TODO: Drupal Rector Notice: Please delete the following comment after you've made any necessary changes.
+    // We are assuming that we want to use the `entity_type.manager` service since no method was called here directly. Please confirm this is the case. See https://www.drupal.org/node/2549139 for more information.
+    $entity_manager = \Drupal::service('entity_type.manager');
     $target_entity_type = $entity_manager->getDefinition($field_definition->getFieldStorageDefinition()->getSetting('target_type'));
 
     // Try to update the default value config dependency, if possible.
@@ -601,7 +605,7 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
 
     // Rebuild the array by changing the bundle key into the bundle label.
     $target_type = $field_definition->getSetting('target_type');
-    $bundles = \Drupal::entityManager()->getBundleInfo($target_type);
+    $bundles = \Drupal::service('entity_type.bundle.info')->getBundleInfo($target_type);
 
     $return = [];
     foreach ($options as $bundle => $entity_ids) {
@@ -685,7 +689,7 @@ class StyleEntityReferenceItem extends EntityReferenceItem {
 
     // Add all the commonly referenced entity types as distinct pre-configured
     // options.
-    $entity_types = \Drupal::entityManager()->getDefinitions();
+    $entity_types = \Drupal::service('entity_type.manager')->getDefinitions();
     $common_references = array_filter($entity_types, function (EntityTypeInterface $entity_type) {
       return $entity_type->isCommonReferenceTarget();
     });
diff --git a/style_entity.info.yml b/style_entity.info.yml
index 40bfd16..bd0aee0 100644
--- a/style_entity.info.yml
+++ b/style_entity.info.yml
@@ -2,6 +2,7 @@ name: HI Styles
 type: module
 description: Add classes to your nodes, blocks and paragraphs using Style config entities.
 core: 8.x
+core_version_requirement: ^8 || ^9
 package: Other
 dependencies:
   - drupal:field
diff --git a/style_entity.module b/style_entity.module
index 7b45157..120e677 100644
--- a/style_entity.module
+++ b/style_entity.module
@@ -1,5 +1,6 @@
 <?php
 
+use Drupal\field\Entity\FieldConfig;
 /**
  * @file
  * Main file of bcbsmn_custom module.
@@ -56,7 +57,7 @@ function style_entity_preprocess_node(&$variables) {
 function style_entity_get_styles($entity, &$variables) {
   $fields = $entity->getFieldDefinitions();
   foreach ($fields as $delta => $field) {
-    if ($field instanceof Drupal\field\Entity\FieldConfig) {
+    if ($field instanceof FieldConfig) {
       /** @var Drupal\field\Entity\FieldConfig $field */
       if ($field->getType() == 'style_entity_reference') {
         $style_field = $delta;
