I am answering my own question for other people's reference. For some reason the font color of the Jfoenix combobox wasn't editable in css. So what I did instead is used the standard combobox provided and simply changed the css to make it look like the Jfoenix combobox.
This is the css style I used for combo-box:
.combo-box,
.choice-box {
-fx-font-size: 12px;
-fx-prompt-text-fill: black;
-fx-text-fill: black;
-fx-background-color: none;
-fx-border-width: 0 0 1 0;
-fx-border-color: #bdbdbd;
-fx-padding: 0 0 3 -8;
}
.combo-box .list-cell:filled:selected .text,
.choice-box .list-cell:filled:selected .text {
-fx-fill: black;
}
.combo-box .arrow,
.choice-box .arrow {
-fx-background-color: #4d4d4d;
}