I know that there is a theme for dark but that is for the mail list, not the actual email itself. Is it possible to have a dark theme for the emails when you read or compose them?
Sorry, I don't think it's possible. I even got tired of this in Gmail and Outlook.com, so I tried to do this in Thunderbird (I know, way off topic, but it's pretty open and plugin-friendly, which is why I mention it), and couldn't get it to do it.
Sadly, most emails are sent as black text on a white background, not unlike a normal letter. Outlook is simply displaying the email as it was sent. As a programmer who uses green-on-black, I'd love to see the ability to inverse the colors in Outlook, or set color defaults.