Are you getting any specific errors, like 403 or a blank layout after importing?

Your forum might start directing visitors to inappropriate websites, or your site might be blacklisted by search engines like Google for distributing malware. Data Theft:

MyBB 1.8 themes are essentially packages that contain files and instructions that change the appearance of a MyBB forum. These themes can modify various aspects of a forum, including the layout, colors, fonts, and graphics. MyBB 1.8 themes are designed to be easy to install and use, allowing users to quickly and easily change the look of their forums.

A MyBB core update can easily break a nulled theme with no way to fix it.