What you're seeing is an
internal border that's fixed in the control's template at BorderThickness="2,2,2,2" and colour white. You can only change it by changing the control's template.
This works:
InitializeComponent();
Loaded += (s, e) =>
{
Border b1 = TreeHelper.FindChild<Border>(TheCalendar);
Border b2 = TreeHelper.FindChild<Border>(b1);
b2.BorderBrush = Brushes.Transparent;
};
It shows a bit of code added into the constructor of the window containing the calendar control. You could change b2.BorderThickness to zero rather than change the colour, if you prefer. Implementation of the TreeHelper class is left as an exercise for the reader.