inject css inline

gulpfile.js

//install
//npm i gulp-inject group-array@0.3.1
var inject = require('gulp-inject');

// inject cssinlining
gulp.task('inject:cssinlining', function() {
    return gulp.src('./app/*.html')
        .pipe(inject(gulp.src('app/css/header.min.css'), {
            starttag: '<!-- inject:cssinlining -->',
            transform: function(filePath, file) {
                return '<style>' + file.contents.toString('utf8') + '</style>'
            }
        }))
        .pipe(gulp.dest('./dist'));
});