I guess it's that time of year again, when I change the editor I use to write. The last editor I was using, LightPaper, hasn't seen any updates in the past year and a half, which is a bummer, because I really think it has potential, and it's already a pretty good app; it just didn't fit my needs/desires. I know, I'm spoiled when it comes to editors, but I'm also a sucker for clean, dark themes.
Recently, I stumbled across Caret, which has everything I look for: in-line Markdown highlighting, syntax assistance, live and customizable (through CSS) preview, an easy-to-the-eyes dark theme and a great, language-aware, syntax theme for code blocks.
Check the full features list here, and maybe give the app a try as well. It has some rough edges here and there, but I definitely think it's worth it.