Example of how to add custom classes to some of the EE buttons, The first function adds a 'qbutton' class to the ticket selector submit button, the second adds a 'qbutton' class to the SPCO checkout buttons

example.php

<?php //Please do not include the opening PHP tag if you already have one.

function tw_add_class_to_ts_button_with_js(){
	wp_add_inline_script( 
		'ticket_selector',
		'jQuery( document ).ready(function($) {
			jQuery(".ticket-selector-submit-btn").addClass("qbutton");
		});'
	);
}
add_filter('wp_enqueue_scripts', 'tw_add_class_to_ts_button_with_js', 11 );

add_filter('FHEE__EE_Form_Input_Base___construct__input_args', 'tw_ee_custom_button_class', 10, 2);
function tw_ee_custom_button_class($input_args, $input_obj) {
	if( 
		!empty($input_args['html_class']) 
		&& strpos($input_args['html_class'] , 'spco-next-step-btn') !== false
		&& $input_obj instanceof EE_Submit_Input
	) {
        $input_args['html_class'] = $input_args['html_class'] . ' qbutton';
	}

  return $input_args;
}