Use colors more similar to the Qt theme for GTK2 scrollbars

The scrollbar should use View Inactive Text/View Hover Decoration colors and a transparent version of the View Foreground color as it does for the Qt style. This fixes the scrollbar being "inverted" (see comparison below) and instead uses the mouseover the

Now (Qt: left to right, unfocused, unfocused mouseover bar, focused, focused mouseover bar, and below GTK: unfocused, focused)

This is a comparison of the Qt version (left to right: unfocused, unfocused mouseover, focused, focused mouseover), the GTK2 theme this MR creates (left to right: unfocused, focused; it doesn't seem possible to make it more accurate to the Qt behavior) and the current upstream GTK2 behavior (same as above) for two different themes: Breeze and Hello Dark. It's way more obvious for the latter that something is different because the colors are swapped.

I'd say this is close enough, and an improvement over what was before.