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:

How to add Custom CSS?