A snippet to add some custom CSS to the Invoice HTML output

tw_ee_AHEE__EE_Html_Messenger_main_wrapper_template_head.php

<?php //Do not include the opening PHP tag if you already have one

// This snippet forces the text to be black on the print/swtich buttons shown on the the invoice. 
// iOS is setting white text on a white background so this foces the buttons to be visible.
add_action('AHEE__EE_Html_Messenger_main_wrapper_template_head', 'tw_ee_AHEE__EE_Html_Messenger_main_wrapper_template_head', 10, 1);
function tw_ee_AHEE__EE_Html_Messenger_main_wrapper_template_head($message_type){ 
	if($message_type instanceof EE_Invoice_message_type) {
		echo '<style type="text/css">input.print_button{ color:#000!important }</style>';
	}
}