Set line-numbers background for classic and tango

To better distinguish the text from the line numbers.

 #e5e5e0 seems a good value. A gray too dark is ugly, and a gray too
light is difficult to see. The blue component (e0) is a bit darker, like
for the other gray colors defined in classic.

Another possibility is to have a lighter gray background, with a
vertical line (same colors as the right margin). Or just a white
background with a vertical line.

https://bugzilla.gnome.org/show_bug.cgi?id=733606