I am also having the same problem. I am using Opera 36.0.2130.65 on Windows 10 (64 bit). I have tried using
background-color: #000000;
background-color : rgb(0, 0, 0);
background-color : rgba(0, 0, 0, 0);
and none of them work. Ironically, setting the text color inside the select box works.