scrollbars: skinnier than gnome apps and also no boundary

[rik-shaw]

There are 2 usability issues with the arc-firefox-theme scrollbars when compared to the Arc GTK theme scrollbars (at least using Ubuntu 15.10, Gnome 3.16).

First, the Firefox scrollbars:

Second, the Nautilus scrollbars (using Arc Darker theme):

Here are the 2 problems:

1. The firefox gray scrollbar is maybe 2 pixels skinnier.
2. The firefox "scrollbar boundary" is not shown. What I mean is the line to the left that shows the user the edge of the scrollbar box. Without that they don't realize they have a few pixels of "grace" when trying to click exactly on that "too skinny gray bar".

Thanks for a great theme.

UPDATE: these "too skinny scrollbars with no boundary" exist in Thunderbird as well (NOT using the arc-firefox-theme), so it must be an issue to report against the generic Arc theme instead of specifically here. Here is a link to the Arc Theme issue filed: horst3180/arc-theme#324