We can use the Appearance -> customize -> Additional CSS and add CSS. We will be able to immediately see what it looks like in the preview window to the right.
Here I have added code with a background color that affects the body tag in the about page (page-id-44).
NB! If one adds code directly to a parent theme files this code will be erased upon an update of the theme. Be sure to add custom code to a child theme. As we have more control over the child theme. In the above example we see the parent theme Blocksy. Which I am using on this site. Notice the notification. WordPress gives the suggestion to add CSS through the customize instead of in the style.css file. I want to add CSS and Php code, so I will instead go with the third option.
Adding custom code into an external plugin.
Adding code to a plugin makes it independent of the theme that is in use. As switching a theme the custom code added through the Customize or in a child theme style.css will be lost and not brought over to the new theme. Adding the code into a plugin makes the code more secure.
Two plugin options are: