A common issue that can occur when installing a WordPress themes is “The package could not be installed. The theme is missing the style.css stylesheet.” error message being displayed when uploading or activating the theme.
You will need to extract/unzip the theme package first that you’ve download.
The package inside including ‘parent theme’, ‘child theme’, demo data and documentation.
Make sure you are uploading only the installable theme (that inside the package you download which is called
theme-name.zip) through your WordPress admin section.
When you have uploaded the theme, you will need to activate the theme itself.