How to Duplicate a Child Theme and Make Changes to the Duplicate Only

Hello,

I am running a Child Theme of Highend Theme 3.5.1.3 on WordPress 5.2.1.

My Child Theme has a custom content width of 960px. I would like to duplicate my Child Theme and try changing the content width to Fullwidth 100%. However, I have a lot of custom CSS in my current Child Theme which I do not want to lose.

How can I duplicate my Child Theme and then make changes to the Duplicate Child Theme, without modifying the original Child Theme? I hope this makes sense… 🙂

Please advise. Thanks.