A theme based on the Breeze theme for Plasma KDE, using the default light colors. Icons from Tela (a version with default Breeze icons can be easily made if there is some request)
You can clone the repository on github
git clone https://github.com/paulatz/Mailspring-Theme-Breeze-Light
or download the zip package and decompress it anywhere. In any case, just use the Add Theme of Mailspring and select the theme directory.