MarkdownStyle.largeFromTheme(ThemeData theme)

Creates a MarkdownStyle from the TextStyles in the provided theme. This style uses larger fonts for the headings than in MarkdownStyle.defaultFromTheme.

Source 

MarkdownStyle.largeFromTheme(ThemeData theme) : super (
  a: new TextStyle(color: Colors.blue[500]),
  p: theme.textTheme.body1,
  code: new TextStyle(
    color: Colors.grey[700],
    fontFamily: "monospace",
    fontSize: theme.textTheme.body1.fontSize * 0.85
  ),
  h1: theme.textTheme.display3,
  h2: theme.textTheme.display2,
  h3: theme.textTheme.display1,
  h4: theme.textTheme.headline,
  h5: theme.textTheme.title,
  h6: theme.textTheme.subhead,
  em: new TextStyle(fontStyle: FontStyle.italic),
  strong: new TextStyle(fontWeight: FontWeight.bold),
  blockquote: theme.textTheme.body1,
  blockSpacing: 8.0,
  listIndent: 32.0,
  blockquotePadding: 8.0,
  blockquoteDecoration: new BoxDecoration(
    backgroundColor: Colors.blue[100],
    borderRadius: new BorderRadius.circular(2.0)
  ),
  codeblockPadding: 8.0,
  codeblockDecoration: new BoxDecoration(
    backgroundColor: Colors.grey[100],
    borderRadius: new BorderRadius.circular(2.0)
  )
);