From 1.5.4 on there is a CSS class .qa-a-count-selected added to the answer count field.
You could use CSS3 to add "best answer selected" to the answer field using:
.qa-a-count-selected:afte.qa-q-list-item {
position:relative;
}
.qa-a-count-selected:after {
font-size:10px;
color:#FFA540;
content:"Best selected";
position:absolute;
left:153px;
top:95px;
width:35px;
}
OR as stated in the question to change the background color:
.qa-a-count-selected {
background:#FFCCCC;
}