Hi Kevin,
The docs provide you insight on what you can style. label and activeLabel
https://docs.mendix.com/refguide/native-styling-refguide#5-1-container
However you can not target nth child as you can do this with CSS, I dont see a way that it is possible, maybe inspecting the widget will give you an sample.
And if worse comes to worse you can build you own widget.
Cheers, Andries