CSS Styling a specific page

That is a URL Query String. It’s still the same page, not a subpage. You can access this in PHP via $_GET and you can use the body_class hook to look for this query string and add a class accordingly. WordPress won’t add classes for query strings, it’s not a normal thing to do.

Here’s an example of how you can use the body_class hook to add an additional class based on the query string:

/**
 * Look for query string and add an additional class to the body
 *
 * @param Array $classes
 *
 * @return Array $classes
 */
function wpse352069_body_class_mods( $classes ) {

    if( isset( $_GET, $_GET['add-new-business'] ) && true == $_GET['add-new-business'] ) {
        $classes[] = 'add-new-business';
    }

    return $classes;

}
add_filter( 'body_class', 'wpse352069_body_class_mods' );

If that query string exists and IS true then it will add the add-new-business class to the body tag. You can use the CSS below to style it:

body.add-new-business .header { background-color: yellow; }