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.