How to install a new VirtueMart theme?
To install a new VirtueMart theme, upload its folder including all the files in the components/com_virtuemart/themes directory.
Check our FTP tutorial for more details on how to upload files here.
Navigate to your Joomla! 3.x administrator area > Components > VirtueMart > Configuration > Templates and pick the new theme from the Select the theme for your Shop drop-down menu.
Save the changes.