Defines the InHighContrast attached property, which is true on the viewer and inherited by every
element of its document while the viewer is in high contrast (IsHighContrast). A control
theme matches it on its own control, ^[(mv|MarkdownViewer.InHighContrast)=True], where a
:high-contrast style would cost memory on every control of its type.
public static readonly AttachedProperty<bool> InHighContrastProperty