Thanks Vidyalakshmi M,
For those interested, I slightly adapted your code to my needs:
SetThemeColors();
RequestedThemeChanged += (s, e) => SetThemeColors();
private void SetThemeColors()
{
Resources["SfSchedulerNormalMoreAppointmentIndicatorColor"] = RequestedTheme switch
{
AppTheme.Dark => Colors.White,
_ => Colors.Black
};
}