Sometimes it will be necessary for you to add some custom CSS to your site to make it exactly how you want.
In WordPress 4.7, WordPress added a core way to add CSS to your site inside the Customizer which you can find in Customize -> Additional CSS.
It should be noted that the CSS added in this area is theme-specific, so if you switch themes, you’ll lose your CSS.
It also doesn’t have the individual page meta box for page specific CSS.
Of course, if you’re already using a Child Theme, you can use the style.css file.
If you’re only adding CSS to your website, a child theme is probably overdoing it a bit – you’re better off using one of the methods above.