If you have to use nulled themes, then find reliable sources. A simple Google search can land you in a cracked website that has unreliable themes and plugins. Most nulled websites are forums, so you also need to check if the user is reliable. Finally, download it and scan using virustotal. If it shows any threats, then better avoid using the theme/plugin unless you understand the threat.
You should take frequent backups and also use security plugins to avoid backdoors and hacks. There are many vulnerabilities and you'll never know the mistakes unless it's too late to recover. So, better be cautious while using nulled themes and plugins.