print css

It depends on the browser someone uses to print.

So you can try adding this to the custom css field in the Coupon Options, but will only work on a Chrome and Safari:

@media print
{
    .print * {-webkit-print-color-adjust:exact;}
}

Unfortunately, that is all I have for you, but here is more about it:

https://developer.mozilla.org/en-US/docs/Web/CSS/-webkit-print-color-adjust