Hi Martin,
I'm using the Solarized Dark color scheme for code examples, but unfortunately some email clients don't display the syntax highlighting correctly. (I'm looking at you MS Outlook, with your Word-based HTML renderer.)
Are you literally seeing a black background, or is it actually a dark green as in the link above? If you can drop me a screenshot and which email client you're using, I can look into what it would take to provide a better experience for you.
Thanks!
Joshua