#1826 Search field shows no text when using a dark GTK theme (white text on white background)

[gcurse]

In scite/gtk/Widget.cxx the search field's background color is hardcoded to white.
I changed it to resetting the background to the color given by the GTK theme.
See https://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-modify-base:

[Neil Hodgson]

Committed as [f03b13].