WordPress at a glance
function is not described

preview_theme() WP 2.6.0

Deprecated from version 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

null. Null. Nothing.

Usage

preview_theme();

Changelog

Since 2.6.0 Introduced.
Deprecated 4.3.0

Code of preview_theme() WP 5.7

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