We have to start at some point!
You'd want to add it to your theme's style.css. The theme editor is the way you can do it from within the WordPress dashboard. You can add the code suggestion at the very bottom of style.css.
The "best practice" way would be to use an FTP editor ( http://codex.wordpress.org/FTP_Clients ) to download the style.css file from your wp-content/themes/THEME-NAME folder, edit it on your computer, then upload it back to the server—keeping a backup just in case. With CSS, it is less critical (since it only impacts the visual part of your site) than when making changes to PHP (e.g. your theme's function.php file. If you have the wrong typo, a bad change to functions.php could create an error that would prevent your site from working).
Generally, you won't want to edit anything via the Plugin editor. Any time a plugin is updated, it would override any edits made.
I hope that helps and makes sense!