How to change default scrollbar of entire page and merely within a div

[aedales]

I am trying to change the default scrollbar of my webpage with the help of a tutorial. The issue is,that the scrollbar changes for the content we put inside a particular div tag and not of the entire page.For example in this demo the scrollbar has changed,but not of the enitire page. Can anyone please help me in changing the scrollbar of the entire page and merely a particular block of contentThanks in advance.