Mailspring is similar to Nylas Mail, but the entire package manager has been replaced to improve performance. Themes work almost the same way, but there are a few changes you’ll need to make to ensure your theme works well in Mailspring. These changes are mostly minor - this should only take a few minutes!

1. Open the theme’s package.json file. Replace this section:

"engines": {
"nylas": "*"
},

…with…

"engines": {
"mailspring": "*"
},

If your theme only works on specific versions of Mailspring, this is where you’d specify the version constraint (eg >=1.0.2)