diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
index 10b5c29a33..b34bc317e1 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
@@ -16,6 +16,8 @@
  */
 class WP_REST_Themes_Controller extends WP_REST_Controller {
 
+	const PATTERN = '[^.\/]+(?:\/[^.\/]+)?';
+
 	/**
 	 * Constructor.
 	 *
@@ -50,7 +52,7 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 
 		register_rest_route(
 			$this->namespace,
-			'/' . $this->rest_base . '/(?P<stylesheet>[\w-]+)',
+			sprintf( '/%s/(?P<stylesheet>%s)', $this->rest_base, self::PATTERN ),
 			array(
 				'args'   => array(
 					'stylesheet' => array(
diff --git a/wp-includes/version.php b/wp-includes/version.php
index a9d1ddc3c5..47a4886c24 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52016';
+$wp_version = '5.9-alpha-52017';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.