Gulpfile sass task with multiple themes or directories using merge streams

gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');
var sourcemaps = require('gulp-sourcemaps');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var plumber = require('gulp-plumber');
var merge = require('merge-stream');
var util = require("gulp-util");

/*************************************
 /* Paths to themes (Or './wp-content/themes/')
 /*************************************/
var themes = {
    root :'./web/app/themes/'
};
/*************************************
 /* Paths of sources
 /*************************************/

var source = {
    folders : [
       themes.root+'my-first-theme',
       themes.root+'my-second-theme',
       themes.root+'my-third-theme'
    ],
    fileType : {
        scss: '**/*.scss'
    }
};

/*************************************
 /* Task Sass
 /*************************************/
gulp.task('sass', function () {
    var tasks = source.folders.map(function(element){
        // Console source folders
        util.log(util.colors.green('Themes: '),util.colors.yellow( element ));
        return gulp.src([
                element + '/assets/scss/style.scss',
                element + '/assets/scss/admin/dashboard.scss'
            ])
            .pipe(plumber({
                handleError: function (err) {
                    console.log(err);
                    this.emit('end');
                }
            }))
            .pipe(sourcemaps.init())
            .pipe(sass({debugInfo: true, errLogToConsole: true}))
            .pipe(sourcemaps.write('.'))
            .pipe(gulp.dest(element + '/css/'));

    });
    return merge(tasks);
});
 
/*************************************
 /* Gulp Watch
 /*************************************/
gulp.task('watch', ['sass'], function () {
    gulp.watch( [source.fileType.scss], ['sass']).on('change', function(file) {
        util.log(util.colors.yellow('SCSS changed' + ' (' + file.path + ')'));
    });
});