If you purchase a legitimate theme or download a free one from a trusted source, follow these steps to install it:
People don't "null" scripts out of the goodness of their hearts. Most nulled themes are modified to include malicious code Data Theft: Mybb Premium Themes Nulled Script
If you are not paying for the product, you are the product. And in the case of nulled scripts, your server is what the hackers are selling. If you purchase a legitimate theme or download