Add third customizable css file to customize rich-text editor (WYSIWYG).
As far as I tested. There are two CSS. To customize App and renderded Markdown. Also there is an option to customize Markdown editor (I think...).
But no customization of rich-text editor (WYSIWIG)? I could possibly play with ZOOM option and do css styling around it. But that just sounds weird, right?
userchrome.css
userstyle.css