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 the section that reads 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")
2. Nylas Mail supported a wide range of "style" folder names - you could put your CSS / LESS stylesheets in "styles", "styleSheets", "stylesheets", etc. In Mailspring, your theme must have a folder called "styles".
3. Nylas Mail allowed you to specify the entry stylesheet in package.json, or create an index.less file at the top level of your theme. Mailspring only looks for your theme's primary stylesheet at `styles/index.less`. If the theme has an index.less file elsewhere, or a specific LESS file listed in package.json, move it there. If you don't specify an index.less file or none exists, Mailspring will load all of the .less / .css files it finds in the styles folder in order.
That's it — enjoy theming Mailspring!