Styles for the drop down of a QComboBox
can be changed by applying relevant changes to QComboBox QAbstractItemView as showed in the following example:
QComboBox QAbstractItemView
{
color:#ccc;
background-color:#333;
border:none;
selection-background-color:#1c1c1c;
selection-color: #ccc;
height: 30px;
}