How do I change style to child themes

Hello folks,

I am watching the video tutorial Creating Child Themes (Part 2). Mark gives some examples about how to modify the child theme appearance by adding code to style.css, but in the version I am working on, 8.9.7, there is a note that states:

NOTE THIS FILE IS NOT USED WITHIN THE THEME
THEREFORE ADDING CONTENT TO IT WILL NOT WORK

How do I add styles?

Thanks,
Marco