Sometimes all you need is a simple plugin to add custom CSS to WordPress websites. Heaven knows there’s no lack of them in the WordPress repository.
I prefer to keep all CSS in the theme’s folder but sometimes I need to add some custom CSS on-the-fly without having to go through the trouble of updating the actual theme code. Maybe the client has reported a visual anomaly or I spotted something off while browsing their website. Either way, it’s much easier to log into the WordPress admin and add the CSS code. Migrating it to the theme can be done at a properly scheduled time.