Unfortunately, Firefox currently has very limited support for adjusting scrollbar widths. I'd love to expand the options available for that when more options are added to Firefox.

As for customizing this page, that's a restriction Firefox has for all add-ons. There are some protected pages (such as internal browser settings pages or Mozilla-owned pages), which add-ons are not allowed to edit. The intention is to prevent add-ons from injecting false or dangerous information on the pages, but it also means that the Custom Scrollbars add-on can't modify the scrollbars on this page.