Unibit is a superset of existing static site generators, if you build a theme according to the Unibit spec then Stackbit will be able to convert (transpile) it into a host of target SSGs.
When we started Stackbit we built Unibit as an internal tool that enabled us create a theme once and convert it to multiple static site generators. When you create a new site on Stackbit using a Unibit theme you can pick any of the target SSGs for the project being generated. While still in beta we're releasing the spec for Unibit and a CLI tool to validate and build Unibit sites.
Unibit thems can currently be transpiled into Jekyll, Hugo & Gatsby sites.
Stackbit supports custom themes built with any static site generator, Unibit is one more option with the side benefit of supporting multiple target SSGs.