Related: tdf#90214 gtk2 use a darkcolor based on the theme

this is similar to e6d6e35320984fe4198314e3ea3a91ead6b41392 for gtk3 where...

Related: rhbz#1287593 issue 8, set a 'dark shadow' color based on the theme

pick something that is supposed to be a large contrast against the bg but
is different from the fg

Change-Id: If2d3a1b485dc5ec6be58d434394499cf5f851d0e
(cherry picked from commit f50270ea5cb20c589edc2111e92412ffff4734ca)
diff --git a/vcl/unx/gtk/salnativewidgets-gtk.cxx b/vcl/unx/gtk/salnativewidgets-gtk.cxx
index 5c70c1e..b9bb56a 100644
--- a/vcl/unx/gtk/salnativewidgets-gtk.cxx
+++ b/vcl/unx/gtk/salnativewidgets-gtk.cxx
@@ -3966,7 +3966,7 @@ void GtkSalGraphics::updateSettings( AllSettings& rSettings )

    // background colors
    Color aBackColor = getColor( pStyle->bg[GTK_STATE_NORMAL] );
    Color aBackFieldColor = getColor( pStyle->base[ GTK_STATE_NORMAL ] );
    Color aBackFieldColor = getColor( pStyle->base[GTK_STATE_NORMAL] );
    aStyleSet.Set3DColors( aBackColor );
    aStyleSet.SetFaceColor( aBackColor );
    aStyleSet.SetDialogColor( aBackColor );
@@ -3975,6 +3975,10 @@ void GtkSalGraphics::updateSettings( AllSettings& rSettings )
    aStyleSet.SetWindowColor( aBackFieldColor );
    aStyleSet.SetCheckedColorSpecialCase( );

    // Dark shadow color
    Color aDarkShadowColor = getColor( pStyle->fg[GTK_STATE_INSENSITIVE] );
    aStyleSet.SetDarkShadowColor( aDarkShadowColor );

    // highlighting colors
    Color aHighlightColor = getColor( pStyle->base[GTK_STATE_SELECTED] );
    Color aHighlightTextColor = getColor( pStyle->text[GTK_STATE_SELECTED] );