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.
One of the reasons you may receive this error is if you have uploaded the incorrect file. You have 2 options for downloading your theme.
Installable WordPress file only. (below)
Upload Via FTP
WordPress files can also be uploaded manually via FTP.
Upload the files to /wp-content/themes/ folder on your server.
Login to your WordPress admin area to change the theme.
Check out the guide on WordPress.org here.