How can I add a custom scrollbar?

Hi!
I would like to add a custom scrollbar with a “smooth” effect, like this : https : // idiotwu .github .io/smooth-scrollbar/

I dont know if it’s possible as I understood we can’t access DOM and html files.
Is there still a way to do it?

Thanks for your answers

First, everything I said below is NOT inside the Editor

Second, you can apply the code in Dashboard > Settings > Custom Code

Third, you can see there is lots of HTML scroll bar example on Google. My favorite: How To Create a Custom Scrollbar

Note: you have to “make” all the css, is into one piece of code so you can add into the settings. Smooth effects mostly need JS, so make sure the script is also in one piece of HTML code