When you create a new site in a WordPress multisite installation, the created site will use WordPress’s default theme, which most likely will be the newest theme they created. However, you may not like or want this. You can change the default theme a new multisite site uses by adding the following to your wp-config.php file:

define( 'WP_DEFAULT_THEME', 'custom-theme-name' );

Replace custom-theme-name with the name of the theme you want to be the default.