The Coi theme should be installed without any errors, but here are the most common errors and the solutions:
The package could not be installed. The theme is missing the style.css stylesheet.
One of the most common reasons people have issues is because they try uploading the full zip that includes demo content, theme, documentation…etc which displays the “The package could not be installed. The theme is missing the style.css stylesheet.” error. Please make sure you are uploading only the installable theme which is called cislen-installable.zip.
Are you sure you want to do this?
It means you have an upload file size limit. You need to install the theme or plugins via FTP or contact your hosting provider to increase the limit.
White Screen Of Death or PHP Error Notice
A “white screen of death” happens when there is a PHP error on the site but WP_Debug isn’t enabled on the server. So the first thing to do would be to enable WP_Debug so you can read what the actual error is and thus fix the problem.