MarkdownViewer colour overrides cannot be cleared

MarkdownViewer's colour properties (TextColour, HeadingColour, LinkColour, BackgroundColour, and the other styling colours) latch permanently once assigned: the viewer stops following the light/dark theme for that colour, and there is no way to return to the built-in theme-aware default. Assigning the light-mode default colour manually is not equivalent, because the control no longer adapts when the appearance changes.