I am still using the default Octopress 3 theme, but would like to use Jekyll themes The instructions online, though, suggest that I fork a theme, change the _config file, and then add my content.
It seems like, since the themes should just be sass and _layout, I should be able to install those files and have a new theme.
I haven't seen clearer examples or documentation on that.
Posting here per the jekyll website.
Generally, the best way is to fork the theme, and copy over your content in the _posts folder, and your _config file, as per this answer.
It would also be possible to copy over the theme files into the _sass, css, _includes and _layouts folders, since this is where the theme rests, but this would mean a) more work and b) you can quite easily mess it up. Oh, and the js folder, the index.html file, any other pages not using the page layout...
Ultimately, Jekyll is not like Wordpress; It's not easy to just drop in a new theme.
Forking a jekyll theme just for using it is not a good practice, even github has recognised it and now provides the template type. Forking is good mainly for contributing back to the original.
The easiest way to switch theme on an existing or new jekyll installation is to use the following plugin: jekyll-remote-theme, which is available since November 2017.
The remote_theme method is almost similar to Wordpress usability, instead of clicking around you have to type-in the name of the new theme in the config file. You do not need to fork a theme or touch any of the style files.
This is an elegant solution, because in the long term you prefer the experts (i.e., the theme maintainer) to handle bugs and new version in the theme, while you can focus on your content.
