WordPress nulled themes are very popular as they usually contain free WordPress themes that go for $30 to $50 on themeforest. Nulled in this context is a word used in the WordPress community to represent paid themes that are free.
The legality of using these WordPress themes is indeed interesting, because using these themes without paying is not piracy if it is GPL 2. We can’t speak to other licenses, but under the GPL, users buy open source code and then have the right to give it away for free.
This novel concept, which is at the core of the GPL makes nulled WordPress themes legal to share and distribute free of charge provided it is completely under the GPL 2.
The next part of this question we must consider is if these themes are safe to use for your WordPress site. Unfortunately, the majority of nulled WordPress themes have malware.
This doesn’t mean the original WordPress theme has malware, nulled themes are generally distributed by malware developers. Unless you know where the malware developers put all the malware in nulled WordPress Themes and how to remove it, then don’t use it.
So, all in all the concept of nulled WordPress themes are in the very spirit of the GPL, but it’s been corrupted by malware developers. Until a safe repo comes online that actively removes malware, instead of adding it, we don’t recommend nulled themes.