Here is a WordPress Twenty-Three Child Theme that you can download and add to your website so that you can add custom CSS or PHP code to the child theme.
Install and activate the theme the same way as any other WP theme.
If you also want to copy all style (patterns) from the main Twenty Twenty-Three theme to the Child Theme, then first export the theme settings from the editor:
In the editor click on the three dots (⋮) in the top-right corner, then on Export

This will download a zip archive that contains all your templates.

After installing the Child Theme, simply drop in its folder all files from this exported .zip archive.

Thats it, now you have a Twenty Twenty-Three Child Theme where you can add custom code, but copied all style (patterns) from the Main Theme.

