Creating a Custom CSS code is quite straightforward as long as you are familiar with the basics of HTML and CSS.
You can easily add your custom CSS using the Customizer.
Go to Appearance > Customize > Additional CSS.
Add your CSS code there.
Click on Publish.
If you want a detailed description of adding custom CSS, you can refer to this link: Creating your own custom CSS via Inspect Element
It is shown in the image below: