To upload through your Dashboard, select Appearance and then Themes. Next, select the Install Themes tab at the top of the page and then select the Upload link, right above the search field. Choose your file and select Install Now. Once the theme is fully uploaded and installed, click Activate to activate the WordPress theme.
The theme files will be stored on your server in the wp-content/themes location.
When uploading your theme with the installer, please ensure you are uploading the theme .zip file, not the entire package you downloaded. In this case, you will be uploading sixten.zip.
If you try to install the wrong files, you will get the missing styles.css error. This is the most common error that means you are trying to install an incorrect package.