In this case, please share us your site URL and the admin account of your site via private reply so we can investigate your case and help you out. Also, please point to specific page where we can see the problem. Any screenshots would be welcomed! (you could upload your images into https://imgbb.com/ and share the link here).
P.S: I’m sorry for the mistake, you can add the CSS code into Dashboard > Appearances > Customize > Addition CSS field.