GTK is a free and open-source cross-platform widget toolkit for creating graphical user interfaces. On the maybe less important side, the way Gtk2 and Gtk3 are drawn is a bit different too. I am absolutely certain this is a theme issue. At this point this includes the functionality to use System. #!/usr/bin/env perl use strict; use warnings; use utf8; use Glib::IO; use Gtk3; # Create a new application my $app = Gtk3::Application->new('com. GTK+ 3 is a major new version of GTK+ that breaks both API and ABI compared to GTK+ 2.