Using nulled software is copyright infringement. While a small forum admin is unlikely to be sued for $10,000, you are absolutely at risk for:
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
Let’s be honest about the psychology. Forum owners search for nulled themes for three primary reasons: Using nulled software is copyright infringement
: Nulled scripts can "phone home" to steal sensitive user information, including email addresses, passwords, and private messages. Malware Injection including email addresses