This is basic styling that works in conjunction with the styles that are available as part of the plugin.
*Note: If you style the button CSS it will get over-ridden by the plugin styles… if they are used or chosen. The form background colours and borders will not get overridden.
/*--Newsletter styling--*/
.mc4wp-form {background-color: #4b7ebb; color: #fff; padding: 20px 20px 0 20px; border: 2px solid #3568a6;}