How do you add and configure a Jekyll theme to Github Pages?

[MrSpokes]

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I have been trying to get a pages site for a repository I am making, but I have run into some issues with getting the theme for it. This is my _config.yml file:

theme: jekyll-theme-cayman

It is in the main branch, and the site is set to use main/(root). I also have an index.html file and have been able to get a theme before. What am I doing wrong?

[MegMM]

Ditto for problems setting up a theme for Pages. I'm new to it and the documentation is scattered, slightly disorganized and imprecise. All of the documentation says "navigate to _config.yaml" as if it should be there automatically. But I don't have that. My readme.md file is published and showing up from pages.github.io. But I haven't found any documentation for how to proceed past that.