Markdown is concerned with content, not presentation:
The idea for Markdown is to make it easy to read, write, and edit prose. HTML is a publishing format; Markdown is a writing format. Thus, Markdown’s formatting syntax only addresses issues that can be conveyed in plain text.
You aren't giving us much detail about how you are using Flutter to display your content, but the solution won't be a pure Markdown one. A common approach when rendering to HTML is to modify the stylesheet being used (because CSS is about presentation).