Although you can change almost every aspect of WordPress themes, to design your site exactly how you want it, you may occasionally need to add some custom CSS.
In WordPress 4.7, a new way to add CSS to your site was added inside the Customizer which you can find in Customize > Additional CSS.
However, CSS added in this area is theme-specific, so if you switch themes, you’ll lose your CSS.
That is why I recommend using either a plugin to add custom CSS or a Child Theme.
Add Custom CSS code to WordPress using a plugin
The easiest way is to install Simple CSS
The CSS meta box that is included on each page is another benefit of this plugin. This gives you the option to apply CSS that just applies to that particular page, which can be quite helpful.