How do I make my theme unavailable to others

I will publish the theme I made with Jekyll on github, but I don’t want anyone else to use it because I made the theme privately. How can I prevent this.

Any public repository is accessible to everyone. To have it be accessible only to you, it should be a private repository. At that point, GitHub Actions MAY NOT have access to that repository. (Not sure if you can workaround using special Access Tokens).