Ticket #51182: 51182-2.diff

src/wp-admin/includes/class-theme-installer-skin.php

@@ -206 +206 @@
                 $all_themes         = wp_get_themes( array( 'errors' => null ) );
 
                 foreach ( $all_themes as $theme ) {
-                        if ( rtrim( $theme->get_stylesheet_directory(), '/' ) !== $folder ) {
+                        $stylesheet_dir = wp_normalize_path( $theme->get_stylesheet_directory() );
+                        if ( rtrim( $stylesheet_dir, '/' ) !== $folder ) {
                                 continue;
                         }