The CSS is distributed in the Github repo, ostensibly under an open-source license, which the project maintainer has no right to use, distribute, or relicense.

The CSS is distributed in the Github repo, ostensibly under an open-source license, which the project maintainer has no right to use, distribute, or relicense. It's about as clear a case of copyright infringement as they come. (And that's not even mentioning that reusing someone else's CSS likely requires you to reuse their markup, too.)