Get rid of deprecated functions to customize widget colors
    
    Since commit cc455b7f916110d7cfae6b7af753349e070c9494, the background
    color has not been set correctly for Gtk+ 3.18 (Fedora 23), while it was
    working fine for more recent version 3.20 (Fedora 24). Tracked down to
    some changes in GtkNotebook code which can not be easily backported.
    
    Instead of customizing background and foreground colors, let's just
    stick with the default colors provided by theme.
    
    Related https://bugs.freedesktop.org/show_bug.cgi?id=94276
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>