How do you add Custom CSS to a theme?

Hello, Sorry if this is a dumb question, but I am used to being able to add my custom CSS to any WP theme, and on these themes I can't seem to figure out how or where to do that? There is no editor under the Appearance tab, why is this? Also, I have created child themes with other frameworks in the past, will these themes work the same way?

Thanks so much for any help you can give.