gistfile1.php

<?php
/**
 * Dont Update the Theme
 *
 * If there is a theme in the repo with the same name, this prevents WP from prompting an update.
 *
 * @since  1.0.0
 * @param  array $r Existing request arguments
 * @param  string $url Request URL
 * @return array Amended request arguments
 */
function ea_dont_update_theme( $r, $url ) {
	if ( 0 !== strpos( $url, 'https://api.wordpress.org/themes/update-check/1.1/' ) )
 		return $r; // Not a theme update request. Bail immediately.
 	$themes = json_decode( $r['body']['themes'] );
 	$child = get_option( 'stylesheet' );
	unset( $themes->themes->$child );
 	$r['body']['themes'] = json_encode( $themes );
 	return $r;
 }
 add_filter( 'http_request_args', 'ea_dont_update_theme', 5, 2 );

Does this still work? I can't seem to get rid of that update message on my active theme.

Does this still work? I can't seem to get rid of that update message on my active theme.

Check this article:
https://themetry.com/unwanted-theme-updates/