If you get the error message saying “The package could not be installed. The theme is missing the style.css stylesheet.” , this is most likely because you try to upload the whole zip file you downloaded from ThemeForest. This is not the correct way to install the theme, as this zip file contains not only the theme, but PhotoShop files, documentation, plugins, etc.
You have to unzip the main download zip file and then upload the theme zip only. For more information about installing the theme correctly, you can refer to the “Activating the theme” section of the documentation included or watch the “Installing a Pexeto Theme” videos here.