Long-overdue, but I have a working version! I had to:

• Use the latest Curses::UI (v0.9629) with the fix from rt://50142 (here)
• Use only custom colors --- don't try to combine custom colors with standard colors
• Set the color on the Curses::UI instance, the window, and every widget

Et voila, a curses UI with an orange background and white text (tested on Perl 5.26):

use strict;
use warnings;
use Curses::UI '0.9609';

my $cui = Curses::UI->new( -color_support => 1, -clear_on_exit => 1 );
$cui->set_binding( sub { $cui->mainloopExit() }, "\cC" );

# Colors
my $co = $Curses::UI::color_object;
$co->define_color( 'wh', 999, 999, 999 );    # range [1, 999]
       # For some reason, I don't seem to be able to mix
       # standard colors with custom colors.
$co->define_color( 'orange', 999, 599, 1 );
my $FG     = 'wh';
my $BG     = 'orange';
my @COLORS = (
    ( map { ; $_ => $BG } qw(-bg -tbg -bbg -sbg) ),
    ( map { ; $_ => $FG } qw(-fg -tfg -bfg -sfg) )
);

$cui->set_color_fg($FG);
$cui->set_color_bg($BG);

# Widgets
my $win = $cui->add(
    'main-window', 'Window',
    -border         => 1,
    -title          => "Ce n'est pas une fenetre",
    -titlefullwidth => 1,
    @COLORS,
);

$win->add(
    'help', 'Label',
    -y             => 1,
    -width         => -1,
    -reverse       => 1,
    -paddingspaces => 1,
    -text          => "Howdy!  Press Ctl-C to exit.",
    @COLORS,
);

$cui->mainloop;
$cui->leave_curses;
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