I like the new Editor but the very first version had a not so cool Icon, so I decided to draft a replacement. Sadly Matt Zanchelli was faster and already contributed a new Icon at GitHub.
But that's of course no reason to not show what I've created!
I've attached a full .icns file, so if you want, you can download and use it.
1. open the applications folder
2. right click at MacDown > show package content
3. go to Contents > Resources
4. paste and replace the new icon here (yes you have to replace the old icon otherwise it wont work)