inuyaksa

With this plugin you can add nicescroll to your WordPress site.

WP-Nicescroll

Add nicescroll to your wordpress site, it's a jquery (since 1.5) plugin for nice scrollbars with a very similar ios/mobile style.

anuislam

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

As Nice Scroll

As nice scroll is a jquery plugin for WordPress site. This plugin will create a nice scrollbar for your site.

WP-Nicescroll vs As Nice Scroll