diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php
index 08cba90a23..5dda1a9360 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php
@@ -11,6 +11,15 @@
  * Base Global Styles REST API Controller.
  */
 class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
+
+	/**
+	 * Post type.
+	 *
+	 * @since 5.9.0
+	 * @var string
+	 */
+	protected $post_type;
+
 	/**
 	 * Constructor.
 	 * @since 5.9.0
@@ -18,6 +27,7 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	public function __construct() {
 		$this->namespace = 'wp/v2';
 		$this->rest_base = 'global-styles';
+		$this->post_type = 'wp_global_styles';
 	}
 
 	/**
@@ -75,22 +85,32 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	}
 
 	/**
-	 * Checks if the user has permissions to make the request.
+	 * Checks if a given request has access to read a single global style.
 	 *
 	 * @since 5.9.0
 	 *
+	 * @param WP_REST_Request $request Full details about the request.
 	 * @return true|WP_Error True if the request has read access, WP_Error object otherwise.
 	 */
-	protected function permissions_check() {
-		// Verify if the current user has edit_theme_options capability.
-		// This capability is required to edit/view/delete templates.
-		if ( ! current_user_can( 'edit_theme_options' ) ) {
+	public function get_item_permissions_check( $request ) {
+		$post = $this->get_post( $request['id'] );
+		if ( is_wp_error( $post ) ) {
+			return $post;
+		}
+
+		if ( 'edit' === $request['context'] && $post && ! $this->check_update_permission( $post ) ) {
 			return new WP_Error(
-				'rest_cannot_manage_global_styles',
-				__( 'Sorry, you are not allowed to access the global styles on this site.' ),
-				array(
-					'status' => rest_authorization_required_code(),
-				)
+				'rest_forbidden_context',
+				__( 'Sorry, you are not allowed to edit this global style.' ),
+				array( 'status' => rest_authorization_required_code() )
+			);
+		}
+
+		if ( ! $this->check_read_permission( $post ) ) {
+			return new WP_Error(
+				'rest_cannot_view',
+				__( 'Sorry, you are not allowed to view this global style.' ),
+				array( 'status' => rest_authorization_required_code() )
 			);
 		}
 
@@ -98,13 +118,15 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	}
 
 	/**
-	 * Checks if a given request has access to read a single global styles config.
+	 * Checks if a global style can be read.
 	 *
-	 * @param WP_REST_Request $request Full details about the request.
-	 * @return true|WP_Error True if the request has read access for the item, WP_Error object otherwise.
+	 * @since 5.9.0
+	 *
+	 * @param WP_Post $post Post object.
+	 * @return bool Whether the post can be read.
 	 */
-	public function get_item_permissions_check( $request ) {
-		return $this->permissions_check( $request );
+	protected function check_read_permission( $post ) {
+		return current_user_can( 'read_post', $post->ID );
 	}
 
 	/**
@@ -117,9 +139,9 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	 * @return WP_REST_Response|WP_Error
 	 */
 	public function get_item( $request ) {
-		$post = get_post( $request['id'] );
-		if ( ! $post || 'wp_global_styles' !== $post->post_type ) {
-			return new WP_Error( 'rest_global_styles_not_found', __( 'No global styles config exist with that id.' ), array( 'status' => 404 ) );
+		$post = $this->get_post( $request['id'] );
+		if ( is_wp_error( $post ) ) {
+			return $post;
 		}
 
 		return $this->prepare_item_for_response( $post, $request );
@@ -134,7 +156,32 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	 * @return true|WP_Error True if the request has write access for the item, WP_Error object otherwise.
 	 */
 	public function update_item_permissions_check( $request ) {
-		return $this->permissions_check( $request );
+		$post = $this->get_post( $request['id'] );
+		if ( is_wp_error( $post ) ) {
+			return $post;
+		}
+
+		if ( $post && ! $this->check_update_permission( $post ) ) {
+			return new WP_Error(
+				'rest_cannot_edit',
+				__( 'Sorry, you are not allowed to edit this global style.' ),
+				array( 'status' => rest_authorization_required_code() )
+			);
+		}
+
+		return true;
+	}
+
+	/**
+	 * Checks if a global style can be edited.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param WP_Post $post Post object.
+	 * @return bool Whether the post can be edited.
+	 */
+	protected function check_update_permission( $post ) {
+		return current_user_can( 'edit_post', $post->ID );
 	}
 
 	/**
@@ -146,9 +193,9 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
 	 */
 	public function update_item( $request ) {
-		$post_before = get_post( $request['id'] );
-		if ( ! $post_before || 'wp_global_styles' !== $post_before->post_type ) {
-			return new WP_Error( 'rest_global_styles_not_found', __( 'No global styles config exist with that id.' ), array( 'status' => 404 ) );
+		$post_before = $this->get_post( $request['id'] );
+		if ( is_wp_error( $post_before ) ) {
+			return $post_before;
 		}
 
 		$changes = $this->prepare_item_for_database( $request );
@@ -289,6 +336,34 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 		return $response;
 	}
 
+	/**
+	 * Get the post, if the ID is valid.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param int $id Supplied ID.
+	 * @return WP_Post|WP_Error Post object if ID is valid, WP_Error otherwise.
+	 */
+	protected function get_post( $id ) {
+		$error = new WP_Error(
+			'rest_global_styles_not_found',
+			__( 'No global styles config exist with that id.' ),
+			array( 'status' => 404 )
+		);
+
+		$id = (int) $id;
+		if ( $id <= 0 ) {
+			return $error;
+		}
+
+		$post = get_post( $id );
+		if ( empty( $post ) || empty( $post->ID ) || $this->post_type !== $post->post_type ) {
+			return $error;
+		}
+
+		return $post;
+	}
+
 
 	/**
 	 * Prepares links for the request.
@@ -323,7 +398,7 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	protected function get_available_actions() {
 		$rels = array();
 
-		$post_type = get_post_type_object( 'wp_global_styles' );
+		$post_type = get_post_type_object( $this->post_type );
 		if ( current_user_can( $post_type->cap->publish_posts ) ) {
 			$rels[] = 'https://api.w.org/action-publish';
 		}
@@ -371,7 +446,7 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 
 		$schema = array(
 			'$schema'    => 'http://json-schema.org/draft-04/schema#',
-			'title'      => 'wp_global_styles',
+			'title'      => $this->post_type,
 			'type'       => 'object',
 			'properties' => array(
 				'id'       => array(
@@ -426,7 +501,19 @@ class WP_REST_Global_Styles_Controller extends WP_REST_Controller {
 	 * @return true|WP_Error True if the request has read access for the item, WP_Error object otherwise.
 	 */
 	public function get_theme_item_permissions_check( $request ) {
-		return $this->permissions_check( $request );
+		// Verify if the current user has edit_theme_options capability.
+		// This capability is required to edit/view/delete templates.
+		if ( ! current_user_can( 'edit_theme_options' ) ) {
+			return new WP_Error(
+				'rest_cannot_manage_global_styles',
+				__( 'Sorry, you are not allowed to access the global styles on this site.' ),
+				array(
+					'status' => rest_authorization_required_code(),
+				)
+			);
+		}
+
+		return true;
 	}
 
 	/**
diff --git a/tests/phpunit/tests/rest-api/rest-global-styles-controller.php b/tests/phpunit/tests/rest-api/rest-global-styles-controller.php
index 356380ec85..e99232985b 100644
--- a/tests/phpunit/tests/rest-api/rest-global-styles-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-global-styles-controller.php
@@ -7,6 +7,7 @@
  */
 
 /**
+ * @covers WP_REST_Global_Styles_Controller
  * @group restapi-global-styles
  * @group restapi
  */
@@ -16,11 +17,21 @@ class WP_REST_Global_Styles_Controller_Test extends WP_Test_REST_Controller_Test
 	 */
 	protected static $admin_id;
 
+	/**
+	 * @var int
+	 */
+	protected static $subscriber_id;
+
 	/**
 	 * @var int
 	 */
 	protected static $global_styles_id;
 
+	/**
+	 * @var int
+	 */
+	protected static $post_id;
+
 	private function find_and_normalize_global_styles_by_id( $global_styles, $id ) {
 		foreach ( $global_styles as $style ) {
 			if ( $style['id'] === $id ) {
@@ -48,8 +59,15 @@ class WP_REST_Global_Styles_Controller_Test extends WP_Test_REST_Controller_Test
 				'role' => 'administrator',
 			)
 		);
+
+		self::$subscriber_id = $factory->user->create(
+			array(
+				'role' => 'subscriber',
+			)
+		);
+
 		// This creates the global styles for the current theme.
-		self::$global_styles_id = wp_insert_post(
+		self::$global_styles_id = $factory->post->create(
 			array(
 				'post_content' => '{"version": ' . WP_Theme_JSON::LATEST_SCHEMA . ', "isGlobalStylesUserThemeJSON": true }',
 				'post_status'  => 'publish',
@@ -59,25 +77,147 @@ class WP_REST_Global_Styles_Controller_Test extends WP_Test_REST_Controller_Test
 				'tax_input'    => array(
 					'wp_theme' => 'tt1-blocks',
 				),
-			),
-			true
+			)
 		);
+
+		self::$post_id = $factory->post->create();
 	}
 
+	/**
+	 *
+	 */
+	public static function wpTearDownAfterClass() {
+		self::delete_user( self::$admin_id );
+		self::delete_user( self::$subscriber_id );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::register_routes
+	 */
 	public function test_register_routes() {
 		$routes = rest_get_server()->get_routes();
 		$this->assertArrayHasKey( '/wp/v2/global-styles/(?P<id>[\/\w-]+)', $routes );
+		$this->assertCount( 2, $routes['/wp/v2/global-styles/(?P<id>[\/\w-]+)'] );
+		$this->assertArrayHasKey( '/wp/v2/global-styles/themes/(?P<stylesheet>[^.\/]+(?:\/[^.\/]+)?)', $routes );
+		$this->assertCount( 1, $routes['/wp/v2/global-styles/themes/(?P<stylesheet>[^.\/]+(?:\/[^.\/]+)?)'] );
 	}
 
 	public function test_context_param() {
-		// TODO: Implement test_context_param() method.
-		$this->markTestIncomplete();
+		$this->markTestSkipped( 'Controller does not implement context_param().' );
 	}
 
 	public function test_get_items() {
-		$this->markTestIncomplete();
+		$this->markTestSkipped( 'Controller does not implement get_items().' );
 	}
 
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_theme_item
+	 * @ticket 54516
+	 */
+	public function test_get_theme_item_no_user() {
+		wp_set_current_user( 0 );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/themes/tt1-blocks' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_cannot_manage_global_styles', $response, 401 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_theme_item
+	 * @ticket 54516
+	 */
+	public function test_get_theme_item_permission_check() {
+		wp_set_current_user( self::$subscriber_id );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/themes/tt1-blocks' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_cannot_manage_global_styles', $response, 403 );
+	}
+
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_theme_item
+	 * @ticket 54516
+	 */
+	public function test_get_theme_item_invalid() {
+		wp_set_current_user( self::$admin_id );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/themes/invalid' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_theme_not_found', $response, 404 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_theme_item
+	 */
+	public function test_get_theme_item() {
+		wp_set_current_user( self::$admin_id );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/themes/tt1-blocks' );
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		unset( $data['_links'] );
+
+		$this->assertArrayHasKey( 'settings', $data );
+		$this->assertArrayHasKey( 'styles', $data );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item
+	 * @ticket 54516
+	 */
+	public function test_get_item_no_user() {
+		wp_set_current_user( 0 );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_cannot_view', $response, 401 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item
+	 * @ticket 54516
+	 */
+	public function test_get_item_invalid_post() {
+		wp_set_current_user( self::$admin_id );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/' . self::$post_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_global_styles_not_found', $response, 404 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item
+	 * @ticket 54516
+	 */
+	public function test_get_item_permission_check() {
+		wp_set_current_user( self::$subscriber_id );
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_cannot_view', $response, 403 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item
+	 * @ticket 54516
+	 */
+	public function test_get_item_no_user_edit() {
+		wp_set_current_user( 0 );
+		$request = new WP_REST_Request( 'GET', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$request->set_param( 'context', 'edit' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_forbidden_context', $response, 401 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item
+	 * @ticket 54516
+	 */
+	public function test_get_item_permission_check_edit() {
+		wp_set_current_user( self::$subscriber_id );
+		$request = new WP_REST_Request( 'GET', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$request->set_param( 'context', 'edit' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_forbidden_context', $response, 403 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item
+	 */
 	public function test_get_item() {
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/global-styles/' . self::$global_styles_id );
@@ -100,9 +240,13 @@ class WP_REST_Global_Styles_Controller_Test extends WP_Test_REST_Controller_Test
 	}
 
 	public function test_create_item() {
-		$this->markTestIncomplete();
+		$this->markTestSkipped( 'Controller does not implement create_item().' );
 	}
 
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::update_item
+	 * @ticket 54516
+	 */
 	public function test_update_item() {
 		wp_set_current_user( self::$admin_id );
 		$request = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
@@ -116,17 +260,61 @@ class WP_REST_Global_Styles_Controller_Test extends WP_Test_REST_Controller_Test
 		$this->assertEquals( 'My new global styles title', $data['title']['raw'] );
 	}
 
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::update_item
+	 * @ticket 54516
+	 */
+	public function test_update_item_no_user() {
+		wp_set_current_user( 0 );
+		$request  = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_cannot_edit', $response, 401 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::update_item
+	 * @ticket 54516
+	 */
+	public function test_update_item_invalid_post() {
+		wp_set_current_user( self::$admin_id );
+		$request  = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$post_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_global_styles_not_found', $response, 404 );
+	}
+
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::update_item
+	 * @ticket 54516
+	 */
+	public function test_update_item_permission_check() {
+		wp_set_current_user( self::$subscriber_id );
+		$request  = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_cannot_edit', $response, 403 );
+	}
+
 	public function test_delete_item() {
-		$this->markTestIncomplete();
+		$this->markTestSkipped( 'Controller does not implement delete_item().' );
 	}
 
 	public function test_prepare_item() {
-		// TODO: Implement test_prepare_item() method.
-		$this->markTestIncomplete();
+		$this->markTestSkipped( 'Controller does not implement prepare_item().' );
 	}
 
+	/**
+	 * @covers WP_REST_Global_Styles_Controller::get_item_schema
+	 * @ticket 54516
+	 */
 	public function test_get_item_schema() {
-		// TODO: Implement test_get_item_schema() method.
-		$this->markTestIncomplete();
+		$request    = new WP_REST_Request( 'OPTIONS', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$response   = rest_get_server()->dispatch( $request );
+		$data       = $response->get_data();
+		$properties = $data['schema']['properties'];
+		$this->assertCount( 4, $properties, 'Schema properties array does not have exactly 4 elements' );
+		$this->assertArrayHasKey( 'id', $properties, 'Schema properties array does not have "id" key' );
+		$this->assertArrayHasKey( 'styles', $properties, 'Schema properties array does not have "styles" key' );
+		$this->assertArrayHasKey( 'settings', $properties, 'Schema properties array does not have "settings" key' );
+		$this->assertArrayHasKey( 'title', $properties, 'Schema properties array does not have "title" key' );
 	}
 }