Created a form and add a QComboBox.

Use this styleSheet

QComboBox QListView {
selection-background-color: qlineargradient(spread:pad, x1:1, y1:0, x2:0, y2:0, stop:0 rgba(104, 30, 30, 255), stop:1 rgba(198, 0, 0, 255));
}
QComboBox {
border: 2px solid grey;
border-radius: 3px;
padding: 1px 18px 1px 3px;
min-width: 6em;
}

Open and close the list in the combo box.
The text in the input field is displayed in different colors while the list is shown.
Sometimes the text is black and sometimes it is gray.