Thanks for the sample!

The theme property is a weak reference, which means the calendar doesn't store a reference to it. So you need to store a strong reference to it before you assign it.

So you can do something like this: 

  Theme = _theme = new CalendarThemeDefinition(),

Here is snippet using your code: 

CalendarThemeDefinition _theme;
        public override void ViewDidLoad()
        {
            base.ViewDidLoad();
            _appts = GenerateAppointments(2);

            _dataSource = new CalendarDataSource(_appts);
            _dataSources = new NSObject[] { _dataSource };

            _calendar = new IGCalendarView {
                DisplayBackButton = false,
                DisplayTitleBar = true,
                DayViewInitialScrollStartHour = 11,
                DayViewShowAMPM = true,
                Frame = this.View.Bounds,
                AutoresizingMask = UIViewAutoresizing.FlexibleWidth | UIViewAutoresizing.FlexibleHeight,
                Theme = _theme = new CalendarThemeDefinition(),
                AppointmentDataSources = _dataSources
            };

Hope this helps!

-SteveZ