How specify different file for maven compile-theme goal?

It seems like the maven compile-theme goal only looks for the file styles.scss. We have another scss file (e.g. external.scss) that is not included in the styles.scss and is separate, but is located in the theme directory. Is there a way to specify the file for the compile-theme goal to also compile this file?

We found that the in development mode, Vaadin seems to compile this file when requested.