I’ve confirmed that the above code does work in 1.10.4. You may need to clear your browser cache to see the change. It is also possible that the change is too subtle and you are not noticing the difference. To make it more obvious you can try something like this:
a.Icon.Ticket.preview b, a.Icon.emailTicket.preview b, a.Icon.phoneTicket.preview b, a.Icon.webTicket.preview b, a.Icon.otherTicket.preview b {
font-weight: 700;
font-size: 10px;
font-family: 'Open Sans', sans-serif;
color: #ffffff;
background: #30a400;
}
But what I really recommend is to upgrade to 1.12.x.