Function that outputs second theme url

You can use two inbuilt functions to do this.

1. switch_to_blog
2. get_stylesheet_directory_uri

You can use both in the following way to make a function out of it which would give you the url to the theme. Put the below in the functions.php file

function wpse_get_theme_url( $blog_id = 1 ){
    switch_to_blog( $blog_id );

    $template_url = get_stylesheet_directory_uri();
    restore_current_blog(); // Switch back to the original blog

    return $template_url;
}

Now you can use the above function to fetch the url by using the function and passing the blog id/ sub site id.

$theme_url = wpse_get_theme_url( $blog_id );
// Replace the $blog_id with blog id of your sub sites.