Because this requires a template change, I’d advise that you setup a child theme. This allows you to make code changes that won’t be overwritten when an X update is released. After your child theme is setup, please review how we recommend making template changes in Customization Best Practices.
Then you can add your custom CSS rules into your child theme’s style.css file. Once you setup the child theme and activate it, you can locate the file under (“/wp-content/themes/x-child/style.css”).