When you try to install a new theme, you may encounter this error message: “The package could not be installed. the theme is missing the style.css stylesheet.” This error is specific to the Add Themes module and is unlikely to occur anywhere else on WordPress. A theme’s files contain coding and stylesheets to manage the elements of a WordPress site.
Depending on your version of WordPress, you may need to download and install additional files to complete the process. In most cases, you can install a theme by uploading its zipped file. If you don’t have an FTP account, you can do this through the help center. Once you have the theme zipped, you will need to upload the style.css file to your WordPress website.