function getCSS([files, options, pages, stylesheets]) {
    /* Ignore specified stylesheets */
    if (options.ignoreSheets.length) {
        stylesheets = stylesheets.map(arr => {
            return arr.filter(sheet => {
                return _.every(options.ignoreSheets, ignore => {
                    if (_.isRegExp(ignore)) {
                        return !ignore.test(sheet);
                    }
                    return sheet !== ignore;
                });
            });
        });
    }

    if (_.flatten(stylesheets).length) {
        /* Only run this if we found links to stylesheets (there may be none...)
         *  files       = ['some_file.html', 'some_other_file.html']
         *  stylesheets = [['relative_css_path.css', ...],
         *                 ['maybe_a_duplicate.css', ...]]
         * We need to - make the stylesheets' paths relative to the HTML files,
         *            - flatten the array,
         *            - remove duplicates
         */
        stylesheets = _.chain(stylesheets)
            .map((sheets, i) => utility.parsePaths(files[i], sheets, options))
            .flatten()
            .uniq()
            .value();
    } else {
        /* Reset the array if we didn't find any link tags */
        stylesheets = [];
    }
    return Promise.all([options, pages, utility.readStylesheets(stylesheets, options.banner)]);
}
Get the contents of CSS files.
@param  {Array}   files       List of HTML files
@param  {Object}  options     UnCSS options
@param  {Array}   pages       Pages opened by jsdom
@param  {Array}   stylesheets List of CSS files
@return {Promise<Array>}