Is there an existing issue for this?

• I have searched the existing issues
• I have read the guide to filing a bug

Use case

Sometimes it's helpful to change the theme for the entire subtree, like you can do with the Theme, DefaultTextStyle or IconTheme widgets. It would be helpful to have the opportunity to do the same for the styles of MarkdownBody (changing MarkdownStyleSheet).

Proposal

DefaultMarkdownStyleSheet(
  style: MarkdownStyleSheet(
    p: TextStyle(color: Colors.orange)
  )
  child: Card(
    child: MarkdownBody(
      data: "My text" // this text is orange
    ),
  ),
);