diff -r 7b1b88e27a20 -r 48c4eec2b7e6 wp/wp-includes/theme-previews.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/theme-previews.php	Fri Sep 05 18:40:08 2025 +0200
@@ -0,0 +1,93 @@
+<?php
+/**
+ * Theme previews using the Site Editor for block themes.
+ *
+ * @package WordPress
+ */
+
+/**
+ * Filters the blog option to return the path for the previewed theme.
+ *
+ * @since 6.3.0
+ *
+ * @param string $current_stylesheet The current theme's stylesheet or template path.
+ * @return string The previewed theme's stylesheet or template path.
+ */
+function wp_get_theme_preview_path( $current_stylesheet = null ) {
+	if ( ! current_user_can( 'switch_themes' ) ) {
+		return $current_stylesheet;
+	}
+
+	$preview_stylesheet = ! empty( $_GET['wp_theme_preview'] ) ? sanitize_text_field( wp_unslash( $_GET['wp_theme_preview'] ) ) : null;
+	$wp_theme           = wp_get_theme( $preview_stylesheet );
+	if ( ! is_wp_error( $wp_theme->errors() ) ) {
+		if ( current_filter() === 'template' ) {
+			$theme_path = $wp_theme->get_template();
+		} else {
+			$theme_path = $wp_theme->get_stylesheet();
+		}
+
+		return sanitize_text_field( $theme_path );
+	}
+
+	return $current_stylesheet;
+}
+
+/**
+ * Adds a middleware to `apiFetch` to set the theme for the preview.
+ * This adds a `wp_theme_preview` URL parameter to API requests from the Site Editor, so they also respond as if the theme is set to the value of the parameter.
+ *
+ * @since 6.3.0
+ */
+function wp_attach_theme_preview_middleware() {
+	// Don't allow non-admins to preview themes.
+	if ( ! current_user_can( 'switch_themes' ) ) {
+		return;
+	}
+
+	wp_add_inline_script(
+		'wp-api-fetch',
+		sprintf(
+			'wp.apiFetch.use( wp.apiFetch.createThemePreviewMiddleware( %s ) );',
+			wp_json_encode( sanitize_text_field( wp_unslash( $_GET['wp_theme_preview'] ) ) )
+		),
+		'after'
+	);
+}
+
+/**
+ * Set a JavaScript constant for theme activation.
+ *
+ * Sets the JavaScript global WP_BLOCK_THEME_ACTIVATE_NONCE containing the nonce
+ * required to activate a theme. For use within the site editor.
+ *
+ * @see https://github.com/WordPress/gutenberg/pull/41836
+ *
+ * @since 6.3.0
+ * @access private
+ */
+function wp_block_theme_activate_nonce() {
+	$nonce_handle = 'switch-theme_' . wp_get_theme_preview_path();
+	?>
+	<script type="text/javascript">
+		window.WP_BLOCK_THEME_ACTIVATE_NONCE = <?php echo wp_json_encode( wp_create_nonce( $nonce_handle ) ); ?>;
+	</script>
+	<?php
+}
+
+/**
+ * Add filters and actions to enable Block Theme Previews in the Site Editor.
+ *
+ * The filters and actions should be added after `pluggable.php` is included as they may
+ * trigger code that uses `current_user_can()` which requires functionality from `pluggable.php`.
+ *
+ * @since 6.3.2
+ */
+function wp_initialize_theme_preview_hooks() {
+	if ( ! empty( $_GET['wp_theme_preview'] ) ) {
+		add_filter( 'stylesheet', 'wp_get_theme_preview_path' );
+		add_filter( 'template', 'wp_get_theme_preview_path' );
+		add_action( 'init', 'wp_attach_theme_preview_middleware' );
+		add_action( 'admin_head', 'wp_block_theme_activate_nonce' );
+	}
+}