AutomateWoo - Adding email CSS with a filter. More info at https://automatewoo.com/docs/email/customizing-your-email-design/

functions.php

<?php


add_filter( 'automatewoo/mailer/styles', 'my_automatewoo_custom_email_styles' );

/**
 * @param string $css
 * @return string
 */
function my_automatewoo_custom_email_styles( $css ) {

	// add custom styles to $css string
	$css .= "
	
		.custom-button-1 {
			display: inline-block;
			font-size: 16px; 
			font-family: Helvetica, Arial, sans-serif; 
			color: #ffffff; 
			text-decoration: none; 
			-webkit-border-radius: 5px; 
			-moz-border-radius: 5px; 
			border-radius: 5px; 
			padding: 12px 18px; 
			border: 1px solid #db3236; 
		}
	
	";

	return $css;
}