To understand "nulled," you must first understand how legitimate Opencart themes work.
: Nulled files frequently include hidden scripts (backdoors) that allow hackers to steal customer data, including credit card information and personal details. No Updates or Support
You upload this ZIP via your Opencart admin panel ( Extensions > Installer ), thinking you are getting a beautiful storefront for free. In reality, you are granting the attacker remote access to your server.
While "nulled" themes might seem like an easy way to get premium designs for free, using them is not recommended