WordPress comes built-in with an easy way to add custom CSS (Cascading Style Sheets) in the Customizer.
To add custom CSS code go to your WordPress admin and then Appearance > Customize.
Click on Additional CSS section, place your custom CSS code in the text area and hit Publish.


Note: In the above screenshot
body {
background-color: #ddd;
}
is only provided as an example. You will have to add the CSS needed for your website.