compress css

gulpfile.js

// install
// npm i gulp-concat gulp-clean-css gulp-uncss
var concat = require('gulp-concat');
var cleanCSS = require('gulp-clean-css');
var uncss = require('gulp-uncss');

// concat, clean and remove unused css
gulp.task('compress:css', function() {
    var cssreset = './app/css/reset.css';
    var cssplug = './app/css/plugins.css';
    var cssfonta = './app/css/font-awesome.min.css';
    var cssstyle = './app/css/style.css';
    var csscolor = './app/css/color.css';
    var cssmy = './app/css/my.css';

    return gulp.src([cssreset, cssplug, cssfonta, cssstyle, csscolor, cssmy])
        .pipe(concat('style.css'))
        .pipe(uncss({
            html: ['./app/*.html']
        }))
        .pipe(cleanCSS())
        .pipe(gulp.dest('./dist/css'));
});