[gorm]

It's hard to judge this without understanding how a theme integrates with WordPress and I suspect the author of this post thinks this is just a mater of calling an API or two.

The way it works is by overriding the behavior of WordPress, both integrating in internal filtering of input/output, reacting on actions/events, integrate with the admin interface of WP and calling APIs.

If this is not a derivate work, nothing is.