Code Snippet – Mod

add_filter( 'aforms_get_stylesheet', 'af_custom_stylesheet', 10, 2 );
function af_custom_stylesheet( $url, $form ) {
    // Change the path to the custom stylesheet for form with ID 1
    if ( $form->get_id() == 1 ) {
        $url = get_stylesheet_directory_uri() . '/custom-form-1.css';
    }
    return $url;
}