This Mailspring theme is based on Apple Mail (iMail/iCloud Mail)

I used Jamie Wilson’s (http://jamiewilson.io) Darkside theme as a base template.

Only tested on Windows. My Mailspring community post

https://community.getmailspring.com/t/apple-theme-for-mailspring/5583

Requirements:

Requires Mailspring avatars addon*

https://github.com/Striffly/mailspring-avatars/releases

Note: I am not affiliated with apple at all