The Custom CSS Feature in Jetpack allows you to customize the appearance of your theme without creating a child theme or worrying about theme updates overwriting your customizations.
You can access Jetpack’s Custom CSS by going to: Appearance > Edit CSS. If you’re new to CSS you can follow this beginner tutorial.
But if you already know what you are doing, you can add your CSS here, then preview the changes or save them. When using Jetpack’s Custom CSS, you won’t lose your changes when you update your theme.
However, if you completely change your theme, this CSS will be removed. Luckily, Jetpack’s Custom CSS saves all your past revisions of your custom stylesheet. By clicking on a past revision, you can restore changes from the past by selecting the revision you want and clicking “Restore this Revision.”
Now you can safely test and save custom CSS for your site using Jetpack Custom CSS.