But might be overkill to add this to the core Bootstrap4XPages projects. It will near legal clearance, and will have to be maintained over time. Then, why only this themes as many more exist?

That said, I bet it is a great addition to Bootstrap4Xpages, as a separate project. There are some techniques using a bundle fragment that will contribute the theme files to the main bundle. This is actually fairly easy.
If you're interested in leading and owning this effort, I can help you.