ChaoticSoul's description reads"
Chaotic Soul is a dark two column theme based off of a certain designer’s old website. It offers one widget sidebar area you can customize. It also bears a neat diptych custom header, and supports custom backgrounds.
As this page notes:
you can build something off of a starting point, but you can’t base anything off of anything. Something is always based on something else.
Sure, usage of "based off" is increasingly common, it is nonetheless nonstandard.