Xamarin and CSS Stylesheets

We are developing a mobile app and I've looked at how to incorporate CSS stylesheets into Xamarin. However, my developer is arguing that using CSS isn't wise, as Xamarin doesn't support all the CSS features and he wants to continue to use XAML for the styles.

Can experienced people give me their recommendations, as he is learning and I want to make sure we go down the right track.

Thanks!