Gulp concat and merge

var gulp = require('gulp');
    scss = require('gulp-sass'),
    autoprefixer = require('gulp-autoprefixer'),
    browserSync = require('browser-sync').create(),
    merge = require('merge-stream'),
    concat = require('gulp-concat');


gulp.task('scss', function () {
  var sass, css;
  
  sass = gulp.src('./src/scss/**/[^_]*.?(s)css')
    .pipe(scss({
        includePaths: [
            'node_modules/slick-carousel/slick'
        ]
    }).on('error', scss.logError))
    .pipe(autoprefixer({
    	flexbox: true,
    	grid: true,
    	browsers: ['> 1%', 'last 5 years', 'Firefox > 20']
    }));

    // Add all the external css files
    css = gulp.src('node_modules/jquery-colorbox/example1/colorbox.css');

    // Merge to styles.css and output to src
    return merge(sass, css)
        .pipe(concat('styles.css'))
        .pipe(gulp.dest('./src/'));
});