Manually generate CSS and output to another file besides main.css

Hi. I know Ionic generates CSS for all the .scss files and outputs the final CSS to main.css file.

Basically, I need to this EXACT process but output this css to another file. In the end, what I need is to generate varied CSS files according to different src/theme/variables.scss files.
I just don’t know how to do that manually.

Any tips?
Thank you.