Editor is now CodeMirror

GitHub switched from ACE to CodeMirror on Gist and GitHub. We need to
- Investigate if ACE is active somewhere, otherwise drop the ACE themes.
- Create new themes. Examples: https://github.com/codemirror/CodeMirror/tree/master/theme
- Do a general pass on things not styles by themes (`cm-s-github` is a partial theme that relies on CodeMirror's default theme to some extent)
