Customize sidebar scroller with gradient

[varlevi]

Customize the sidebar scroller to have a background-gradient matching color input. Make it change dynamically in javascript. Keep the scrollbar thing and maybe dulled so that it doesn't pop too much.

[varlevi]

As mentions in PR related to this issue, I've considered better ways to style this scrollbar and hoping to get it to look something like this:

Hopefully, we can release this feature alongside the site redesign planned for v1.0.0.