We are going to add some simple CSS changes to the headers and the page body to see the custom CSS in action. To do this, follow these steps: 

1. Log in to the WordPress Dashboard as an administrator.
2. Click on the Appearance menu.

3. Click the Customize option.
4. Click the Additional CSS tab to see the following screen. The default instructions will be displayed for use. You can click the Close link after reading the instructions:

5. Open the browser inspection tools and find the CSS class for the site title by right-clicking on the site and clicking the inspection tool on your browser. The following screenshot shows the CSS class using the inspection tool of the Chrome browser:

6. Add the following CSS code to the Additional CSS section to style the title of the site:

.site-title{ background: red; padding : 5px;}

This style change will be shown immediately in the header.

7. Click the Publish button to save the changes.

Now, if you visit the home page, you will see the style changes. Similarly, we can visit any page/post and use the customizer to add dynamic CSS for specific posts/pages.