Where do edit a theme’s CSS?

I want to make a change to the way some thing is styled. I edited themes/sam/static/css/style.css (and reran hugo -D) but my changes are not incorporated into the generated style sheet in public/css/

Is there a separate step for editing a theme?

Call Me Sam is a theme that uses SASS.

Don’t know why the author has a minified CSS under /static/ but the source files are under /assets/sass.

You would also need to use the extended Hugo version, since you are going to modify this theme.