init

plugins/theme-directory.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * Plugin Name: Theme Directory
+ * Description: Registers the repo-root themes/ directory as an additional theme root.
+ */
+
+$repo_theme_root = dirname( ABSPATH ) . '/themes';
+
+register_theme_directory( $repo_theme_root );
+
+add_filter(
+	'theme_root_uri',
+	function ( $theme_root_uri, $siteurl ) use ( $repo_theme_root ) {
+		// Core falls back to the raw filesystem path for registered roots it
+		// cannot map to a URL; rewrite that to where nginx serves themes/.
+		if ( $theme_root_uri === $repo_theme_root ) {
+			return $siteurl . '/themes';
+		}
+		return $theme_root_uri;
+	},
+	10,
+	2
+);