There are predefined color schemes you can set in your options panel. If these don’t suffice you’d have to copy the CSS files and modify it to the other colors you’d want. Though I can’t imagine it wouldn’t suffice – as you can see in the style switcher here you have plenty of choices.
I was also interested in the possibility of editing the style for a “dark” theme, but it looks like “body background color” changes the frame and not the content body. Is there any chance you could aim me in the right direction for making for adding CSS to make the body dark and the text light?