Add "slim" style to scrollbar handle

The scrollbar handle is very wide. I would appreciate having a slim variant that expands on hover.

I'd like a setting to make Fluxer add a "slim scrollbar" style to its custom scrollbar.

• The "slim scrollbar" should be slim while not hovered.
• The "slim scrollbar" should expand while hovered and the scroll bar gutter should fill with a suitable background color.
• (optional) The "slim scrollbar" should fade on its own if:

1. The cursor doesn't move within the scrolling view.
2. The cursor exists the scrolling view.
3. Scrolling has stopped.
4. The layout doesn't update. (For example, when a message is sent, the scrollbar will re-appear).

• The "slim scrollbar" should also not reserve space behind itself.

In Linux, Firefox has scrollbars that are slim when not hovered, but expand to regular size when hovered and fill in the scrollbar-gutter with a solid background color. The scollbar handle also fades when resting. (I believe it inherits this from GTK.)