I want to transfer my additional css code to my theme file–?

If you already had a theme, and you have just added some CSS, what you’ll need to do is create a child theme. The child theme will hold a short comment that tells WordPress to reference the parent theme (the original theme you were using), and then it can contain all your custom styles. This way, you and your friend can both use the original parent theme and whenever it’s updated you will not lose any of your additions.

If you didn’t already have a theme, and you have created a full theme including CSS, all you need to do is .zip the files and send the zip to your friend. The friend can then install the theme directly from that zip file.