Custom build settings for App Scripts to compile extra CSS files

With a standard build for an ionic app I have a variables.scss file that imports the theme to be used and defines some variables to be used in the app.

What I want to do is add a file called variables2.scss and whenever I build have the app build both the regular main.css from the standard variables file, and main2.css from variables2.scss,

This allows me to build multiple discrete themes which I can then switch between in my app.

What I don’t know (and this may be really simple) is what extra config I would to have app scripts process the second input file and generate the second output file in addition to it’s standard behaviour.

Any pointers would be greatly appreciated!