Creating a Child Theme for Joomla
What is a Child Theme?
A Child Theme allows you to set up a standalone directory in which you can create copies of a theme's files (called the Parent Theme) and edit it, without the fear of your work being overwritten in future updates.
How Do You Create a Child Theme?
In your Joomla Administrator page, navigate to Extensions > Template Manager. Check the box next to your template and click on the Duplicate button above.
The Duplicate button will create a second copy of the template. This will become the template override while the primary copy of the template remains the master copy. You may then proceed with changing the files on the template copy.
For more information on Joomla's template override, visit this page at Joomla's Documentation -.