Everytime you create a new site on your WordPress Multi Site network. The default theme will always be automatically assigned to it. So if you would like to change that default theme, this is what you will have to do:
1. Open your FTP client;
2. Open the WP config file;
3. Add the following line of code:
define( ‘WP_DEFAULT_THEME’, ‘your-default-theme’ );
4. Change ‘your-default-theme’ with the name of the theme that you would like to be the default.
5. mind that this line is added before WordPress calls the settings with require_once(ABSPATH . ‘wp-settings.php’);
6. save and upload the wp_config file and you are the hero!