Clone the repository:
git clone https://github.com/...Copy the Adwaita folder to the Mailspring packages folder, normally ~/.config/Mailspring/packages.
Restart Mailspsring and select the theme (Edit -> Change Theme...)
If you are using different font than the default Cantarell, you can set the actual font in ui-variables.less.
This theme is based on the Arc theme by drakkar1969.