Hi,
Thank you, interesting theme :)
I will do my best to update some CSS for this theme.
For the submit button, you can use this PHP code:
function my_gwolle_gb_button_class( $classes ) {
// $classes is a string
$classes .= ' wpcf7-form-control wpcf7-submit';
return $classes;
}
add_filter( 'gwolle_gb_button_class', 'my_gwolle_gb_button_class', 10, 1 );