I dropped Mou in favour of LightPaper http://clockworkengine.com/lightpaper-mac which has more features (tabs, edit or preview panels hiding), better themes and customisation imho.
I personally like MarkdownPad (http://markdownpad.com/, windows only),
but for some non-basic features (GitHub Flavored Markdown, PDF export) you have to buy it for $15.
Mou is a great app, it also has a GitHub style live preview which is very nice, but it can be very slow on large Markdown files. I also had some problems with the syntax highlighter on lists.
Yes, I realize there's other themes, but they're terrible too. I find light text on a dark background fine for code, but terrible for writing prose.
For a good Markdown editor, I recommend Mou. It's been my choice for a while now. http://mouapp.com