As Nice Scroll (as-nice-scroll) free WordPress plugin

As nice scroll is a jquery plugin for WordPress site. This plugin will create a nice scrollbar for your site.
After installing and Activating the plugin, go to WordPress Admin Dahsbord and then go to
Appearance > As option
Here you can customize default settings. So customize settings as you choice and see a nice scrollbar at you site front-end.
you can see hear : https://github.com/anuislam/As-nice-scroll
you can see live demo hear : http://as-nicescroll.tk/

All credit goes to nicescroll