I'm giving Fletcher Penny's new application MultiMarkdown Composer a spin. It's great but I like the dark black styling provided by Byword. Luckily, it's dead simple to create a new style sheet for MMD Composer.
Just create the following folder if it does not already exits
~/Library/Application Support/MultiMarkdown Composer/Styles/
Now create a new style sheet and set the colors and formatting to something you like better. If you want to start with my template, you can find it here. Save it as a ".style" file in the folder mentioned above.
Here's what the new style looks like:
UPDATE: If you need a good source for hex color codes, then visit December.com