make a list of all the stylesheets that need to be processed, then start cranking away

Gruntfile.js

/**
     * Generates our CSS files using libsass.
     * Wrapper for the `sass` task, make sure you have installed all the dependencies of the repo.
     *
     * Example: $: grunt libsass
     *          $: grunt libsass:dev
     *
     * @param env
     */
    grunt.registerTask('libsass', 'Builds our CSS', function(env) {
        var done = this.async(),
            dir = require('node-dir'),
            list = {},
            pkg = grunt.file.readJSON('package.json');

        // Override config options for dev.
        if (env && env == 'dev') {
            grunt.config.set('sass.options.sourceMap', true);
            grunt.config.set('sass.options.sourceComments', true);
            grunt.config.set('sass.options.outputStyle', 'expanded');
        }

        dir.readFiles(pkg.paths.styles, {
                match: /.scss$/,
                exclude: /^_/
            }, function(err, content, next) {
                if (err) throw err;
                next();
            },
            function(err, files) {
                if (err) throw err;
                files.forEach(function(val) {
                    list[val.replace('styles', 'css').replace('scss', 'css')] = val;
                });
                grunt.config.set('sass.dist.files', list);
                grunt.task.run(['clean:css', 'mkdir:css', 'sass']);
                done();
            });
    });