Child theme screwed up the formatting

I took the first steps towards creating a child theme (described here – https://codex.www.ads-software.com/Child_Themes) so I could make a few minor formatting changes to the Yoko theme.

I created a new directory in the themes directory, per the instructions, then created the style.css file per the instructions, but didn’t do any customizing below the line that says “customization starts here.”

When I activated the child theme, which contained no customizations, it screwed up the site’s formatting. First off, how can a style.css file that has no customization instructions mess up the formatting? And second, when I create a child theme, do I also have to copy over all fo the files and subfolders that are in the parent theme, and make the modifications to those files?

Thank you!