how to make stylesheet appear in theme editor?

Question

I have a stylesheet in my theme in the css subdir of the theme dir (css/style.css). The question is how to make this .css file appear in the theme editor of the WordPress admin. (I know using the web editor is a bad idea and I never use it, but the client specifically asked for this).

Note: the only css file that shows up by default in my theme editor is 'style.css' of the root theme dir, but this is not what I need.