Wrong (IMHO) color for unfocused treeviews in some default themes

In some themes shipped by gtk-engines and gnome-themes, when you have a
window with multiple treeviews (for example Rhythmbox), it's hard to
identify by color the currently selected treeview. This because both
focused and unfocused treeviews are using a color based on
selected_bg_selected, just with minor changes (darker or ligther).

You can see this issue in Rhythmbox using Clearlooks: the focused
treeview (track list) is just a little lighter then others.

The same behavior is currently used by Clearlooks, ClearlooksClassic,
Crux, Glossy, Industrial[1], Inverted, Mist, Redmond, Simple.

Instead, Raleigh and Glider are using a good (IMHO) behavior: only
focused treeviews use a color based on selected_bg_selected. unfocused
ones use a shade of gray.

So, my suggestion is use a color based on selected_bg_selected for
focused treeviews and darker(@bg_color) for unfocused ones. In the next
days I'll try to check and change gtkrc files.

[1] while the difference between colors is stronger then in Clearlooks