Got it. For some reason the context menu for "create markdown stylesheet" is disabled in 2013 but if you add the file yourself it works. In the solution root make a folder "Solution Items" and a file "WE-Markdown.css"
you should be able to add styles now and it will updates in your markdown preview.
NOTE: i was using version 1.6 Web Essentials for VS2013
EDIT:
As a simple example i am just using a bootstrap cdn include
@import url(http://netdna.bootstrapcdn.com/bootstrap/3.0.3/css/bootstrap.min.css);
Though it seems that the css support is not very advanced and there are some issues with multi line code blocks. there may be a way to fix this with some css overrides