Changing header font sizes/colors in Markdown

I’m looking for a way to change the font size and color of headers in markdown documents in “Source” view. Is there a simple way to do that? I am a stylesheet dope — I read the documentation on how to modify stylesheets and did a search on the forums and on Google and came away confused.

Not beyond that what the preferences have to offer. It’s source view, in the end. Stylesheets are for preview.

@chrillek is correct.
The source is plain text and cannot be styled.

If you aren’t using syntax highlighting, you can set the plain text font in the Editing > Format settings but no other changes can be made beyond that.