Dark theme enhancement

[stefanovesca]

Please add also a dark theme for mainspring based on this amazing theme.

[grand-lotus-iroh]

+1 request as well if you wanted to kill some time and pump one out...

• https://community.getmailspring.com/t/apple-theme-for-mailspring/5583
• https://developer.apple.com/design/human-interface-guidelines/foundations/color