Extract the zipped package downloaded from Wbcom Designs to your desktop. In the extracted package you will find reign-theme.zip which is the WordPress theme file.
You can install the theme one of two ways:
After uploading the theme, you must preview and activate it.
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 a 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.