hidden-theme.php

<?php

/**
 * Ensure that a specific theme is never updated. This works by removing the
 * theme from the list of available updates.
 */
add_filter( 'http_request_args', function ( $response, $url ) {

	if ( 0 === strpos( $url, 'https://api.wordpress.org/themes/update-check' ) ) {
		$themes = json_decode( $response['body']['themes'] );
		unset( $themes->themes->{get_option( 'template' )} );
		unset( $themes->themes->{get_option( 'stylesheet' )} );
		$response['body']['themes'] = json_encode( $themes );
	}

	return $response;

}, 10, 2 );