Since WordPress 4.7, CSS can be added to your WordPress site through the WordPress Customizer.
- On your admin panel navigate to Appearance -> Customize
- In the Customizer side panel Click Additional CSS
- Add your CSS to the Additional CSS area textbox
- When finished, click the “Save & Publish” button on the top of the side panel
This will only work on WordPress 4.7 or greater. If you’re not and you refuse to upgrade you can add custom CSS using this plugin: https://wordpress.org/plugins/simple-custom-css/