Gives element the control theme with the resource key resourceKey,
such as MarkdownCodeBlockTheme, so that a renderer's own control looks like the built-in one
it stands for. The built-in look of the document's controls comes from these themes, not from
styles on their markdown-* classes. A key that no theme defines yet gives an empty theme,
which the viewer replaces once one does. Replaces the theme the control's type would otherwise
get, so it is meant for plain panels, borders, text blocks and inlines.
public void ApplyTheme(StyledElement element, string resourceKey)