You’re seeing that error because you tried to install the theme via the Plugins section of the WordPress admin. To install a theme follow these steps:
- Log in to the WordPress admin and go to Appearance > Themes.
- Click “Add New” and then “Upload Theme” and select the zip file you downloaded. Then click “Install Now”.
- Once it has installed go to the Themes section, hover over the newly installed theme and click “Activate” and that’s all there is to it.
If you go to Appearance > Customize you will find various settings that will allow you to customize your site.