I spent some of today having a look at the Minima theme on GitHub.

Minima Theme

As this is the theme that all new Jekyll sites have included it seemed like a good place to start. I think I now get how the themes have the required layouts and styling to use for the packaging of content, in this case at least. By adding new content (such as this post) I guess I will be able to see how it is handled by the theme and any susequent ones I change to in the settings.

Primarily it seems to me that themes can be loaded onto a Jekyll website and still be overidden by adding folders with the leading underscore that match up with the ones in the themes. Then any files in the new folders that have the same name as ones in the theme will be used in preference to the ones in the themes.

Minima is very basic but some of the other ones I have seen do appear to have some good stying and functionality. Eventually I will see if I can get these to work here instead of Minima.