Regarding customizing a theme.
It isn’t so maintainable to copy an entire theme just to change a few lines. You forget what you wrote and what comes with the theme and you won’t understand how all the files fit together.
The Jekyll approach is that if you configure your site to use a theme and then add a file to your own repo to override.
For example if is a theme has 3 layouts
- default.html
- post.html
- page.html
And I want to change post.html to maybe shown an icon for post heading.
I copy the whole post.html layout file to my project as
_layouts/post.html
And then I make my updates to the file. I don’t have to add the other layouts. I don’t even need an includes folder or layouts folder in my repo if those all come from the theme.
If I want to add a new layout, I just had author.html for example to my layouts folder.
Themes sometimes provide instructions of how to extend things like CSS so you can just add your own code to one file and have the theme CSS files of 100s of lines get quietly loaded for you.