Gtk 2 theme colors
Do you use show() or show(argc,argv) when showing the main window?
If the latter, that might explain it.
If you don't want the behavior, try the former -- calling show() with
no arguments -- and see if that makes it consistent.
You could call
Fl::scheme("gtk+");
at the beginning of your programs to get a Gtk scheme. I don't know,
whether this takes the colours from your system, but anyway this will
look gtk like on every window manager.