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
Release Date - 21 February, 2016
*Fix some bug.
Release Date - 9th August, 2015
*Include some html, css.