Navigate to StoreFronts and then click on the Themes tab as shown below.
Hover over the Image and click on the duplicate icon:
In the dialog that appears enter the name of the new theme folder and click duplicate. In this example we have entered "MrTeas_Dev".
Theme duplication also supports StoreFront-to-StoreFront duplication of the theme. This allows you to work on a theme in a development sandbox, then duplicate it to your production StoreFront, preview it, and then activate it.
Duplicating in progress:
CONGRATULATIONS, you just duplicated a theme!
Previewing a new theme before activating it for customers
Select a different theme
Delete a theme
All Themes - Developer Tutorials