Dark Mode Documentation

**Is your feature request related to a problem? Please describe.**

I have recently embedded this excellent editor (thank you!) into a website, but now have a directive to support dark mode. So I've begun working in dark mode. Most of the editor looks OK when I override some specific content:

`.CodeMirror{color:#ccc;border-color:#333;background-color:inherit}`

And to get the cursor working I used

`.cm-s-easymde .CodeMirror-cursor{border-color:#ccc}`

The only thing I don't have working is anything with fullscreen or preview modes; all those seem to set the background color.

**Describe the solution you'd like**
Just some documentation detailing my adventure so nobody else has to wander. Thank you!

No file or test is named. Start by reading the existing documentation and checking the fullscreen and preview modes referenced in the issue, along with the provided .CodeMirror and .cm-s-easymde .CodeMirror-cursor overrides. Done means adding a concise dark-mode documentation section that explains the required styling for those modes.

Written by the indexing model from the issue text.