To add custom CSS there are several methods:
First, you need to make sure you have a child theme installed. Read how to install child theme.
Once you have it installed, open style.css and paste the CSS code inside it. WordPress has a built-in code editor, so you can access WordPress > Appearance > Theme Editor , select Rey Child and after the page refresh, click on Stylesheet (style.css) eg: https://d.pr/i/4aOSBu .
Please access Customizer (either in backend Appearance > Customize, or in frontend Customize button in top bar), and access Additional CSS panel.
You can paste your CSS in the code editor.
Do know however that this method is not quite recommended because this CSS is saved into the database.