I used the IE debugger and found a workeraround but it does not seem really clear about the reason why this color is chosen.
Just need to add that stuff below to the styles.css
.nav .open > a, .nav .open > a:hover
{
color: #ffffff;
background-color:#5f5f5f;
}
.nav .open > a:focus
{
color: #ffffff;
background-color:#5f5f5f;
}