Cascading Style sheets, also known as CSS, is a style sheet language used to define appearance, design, and formatting HTML.
WordPress themes, contains a CSS file named style.css in which you can set the rules to format your WordPress site.
You can add Custom CSS while using our themes by going to WordPress Dashboard > Appearance > Customize > Additional CSS, and from there inserting your line of code, within the text input field. Please remember to safe those changes and reload the browser in order to see the modifications on your site. 🙂
In case that you would like to learn more about CSS, I highly recommend using the free tutorials at W3 School. Here, you can find the basics of selectors (how to target the right elements on the page), and Properties (how to change the elements of the page).