Make scroll bar styles consistent throughout extension

Problem:
Throughout the extension we have multiple styles of scroll bars. Browser default and custom. Custom scroll bars don't consistent across operating systems (our custom scroll bars apparently look really bad on linux).

Steps:
1) Audit the various scroll bars throughout the current extension
2) Decide if we're going to go with a default or custom scrollbar
3) Make updates to scrollbars based on #2

Start by auditing the various scrollbar implementations throughout the extension and comparing the browser-default and custom approaches across operating systems, especially Linux. The work is done when one approach is chosen and scrollbar styling is updated consistently throughout the extension.

Written by the indexing model from the issue text.