WordPress at a glance
function is not described

preview_theme() WP 2.6.0

Deprecated in 4.3.0. It is no longer supported and can be removed in future releases. It is recommended to replace this function with the same one.

Start preview theme output buffer.

Will only perform task if the user has permissions and template and preview query variables exist.

No Hooks.

Return

Nothing (null).

Usage

preview_theme();

Code of preview theme: wp-includes/deprecated.php VER 5.0.3

<?php
function preview_theme() {
	_deprecated_function( __FUNCTION__, '4.3.0' );
}