Upload via FTP
Connect to your website host using your FTP client.
Go to your default WordPress themes folder (/wp-content/themes/) and upload the non-zipped theme folder into that folder (/wp-content/themes/).
After uploading the theme, you must preview and activate it via WordPress Dashboard.
Missing stylesheet issue
While installing the theme manually, you may see the “The package could not be installed. The theme is missing the style.css stylesheet.” message.
Don’t worry. If you see this message, it means you’ve uploaded the wrong theme’s file (actually, you’re trying to upload the main theme’s file containing the theme, its plugins, documentation, etc.)
Please, open this .zip file, find the theme’s .zip file in it, and use it instead.