This is a simple style with normalized defaults, it should work on anything from a desktop, to a phone.
Add this to the top of your markdown file
<link href="https://raw.github.com/clownfart/Markdown-CSS/master/markdown.css" rel="stylesheet"></link>
With Markdown CSS, I decided to base all the size off the body font-size. Simply changing the body font size will also change the heading's size, body width, etc.
Since the CSS also uses media queries, it might also be necessary to change the body font-size for your purposes. I do not have any cool phones or tablets, so the extent of my testing was done by resizing my browser window.
Also included is a printing media query. It has not been tested yet, and mainly uses HTML5 Boilerplate defaults.