Finally after a complete headache and several tries. My code below works
div.RadComboBoxDropDown_Metro .rcbHovered
{
border: 1px #7FB8FF !important;
background-color: #7FB8FF !important;
}
.RadComboBox_Metro .rcbFocused .rcbArrowCell, .RadComboBox_Metro .rcbFocused .rcbReadOnly .rcbInputCell, .RadComboBox_Metro .rcbFocused .rcbReadOnly .rcbArrowCellHidden {
background-color: #7FB8FF !important;
border: 1px #5B85B7 !important;
}