Atom syntax theme based on MDN code samples
Even though this is a light syntax theme, I think it also works well with dark UI themes. Here's an example paired with the One Dark UI theme:
$ apm install mdn-syntax
Or Settings → Install → Select
Themes → Search for
Note that this is a derivative of the beautiful Solarized Dark Syntax theme, modified to use colors from MDN.
And, of course, the ever-useful MDN docs with plenty of code samples can be found here: https://developer.mozilla.org/en-US/docs/Web