Use the theme customizer
Within the past few years, WordPress added a feature called the “customizer.” With this, built-in support for custom CSS was released.
To add new CSS to your site, go to Appearance > Customize > Additional CSS.
If you’re using an older plugin, like “Simple Custom CSS,” we recommend moving that CSS, then deleting the plugin.
Put it in your child theme
When you have large amounts of CSS, we recommend putting that in an external file, such as your theme’s style.css file. This can reduce the size o the HTML response and actually improve render times.