also compile and bundle vendor CSS

gulp.d/tasks/build.js

@@ -96,7 +96,7 @@ module.exports = (src, dest, preview) => () => {
     // NOTE use this statement to bundle a JavaScript library that cannot be browserified, like jQuery
     //vfs.src(require.resolve('<package-name-or-require-path>'), opts).pipe(concat('js/vendor/<library-name>.js')),
     vfs
-      .src('css/site.css', { ...opts, sourcemaps })
+      .src(['css/site.css', 'css/vendor/*.css'], { ...opts, sourcemaps })
       .pipe(postcss((file) => ({ plugins: postcssPlugins, options: { file } }))),
     vfs.src('font/*.{ttf,woff*(2)}', opts),
     vfs