Source

File: wp-includes/class-wp-customize-manager.php

public function get_document_title_template() {
		if ( $this->is_theme_active() ) {
			/* translators: %s: Document title from the preview. */
			$document_title_tmpl = __( 'Customize: %s' );
		} else {
			/* translators: %s: Document title from the preview. */
			$document_title_tmpl = __( 'Live Preview: %s' );
		}
		$document_title_tmpl = html_entity_decode( $document_title_tmpl, ENT_QUOTES, 'UTF-8' ); // Because exported to JS and assigned to document.title.
		return $document_title_tmpl;
	}