wp-includes/theme-previews.php:42Adds a middleware to apiFetch to set the theme for the preview.
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'] ) ), JSON_HEX_TAG | JSON_UNESCAPED_SLASHES ) ), 'after' );}Introduced in 6.3.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/theme-previews.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.