In the backend of your website, place your cursor on the “Appearance,” button.
Choose “Customize” from the options that appear.
Note that if you are already signed into your WordPress account, you will have a black band that has the “Customize” option in the front end of your website.
Clicking on the “Customize” link takes you to an editing panel, that has a front-end view of your site to your right and customization options to your left.
Scroll down and click on the “Additional CSS” option.