I already looked at that Gtk link (in fact I mentioned it in my question), but it doesn't really help me. I tried to adapt the example to work with a label, but $label->window returns undef so it dies.

      my $l = new Gtk::Label( "Success" );
      my $colormap = $l->window->get_colormap();
      my $gdk_color = Gtk::Gdk::Color->parse_color( "green" );
      $gdk_color = $colormap->color_alloc( $gdk_color );
      $l->window->set_background( $gdk_color );
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Looking at it a bit closer, it seems to expect that the widget is already added to a realized window. That's unfortunate, since I'd like for my module to return a Gtk::Table that can then be added to a window at will.

As for the theme remark: I would compare what I'm working on to a web browser in that regard. There's some general UI that should be affected by themes (cfr. the browser chrome), and then some content that should be shown a certain way no matter what the theme (cfr. a webpage in the browser)

take a look at the perl-gtk2 class docs. Label is a subclass of widget, which provides the get_colormap function. In your above code change:

my $colormap = $l->window->get_colormap();
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to:

my $colormap = $l->get_colormap();
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I have not used this but it looks right from the docs.

That does get me a little further, but now it still fails on the last line:

      my $l = new Gtk::Label( "Success" );
      my $colormap = $l->get_colormap();
      my $gdk_color = Gtk::Gdk::Color->parse_color( "green" );
      $gdk_color = $colormap->color_alloc( $gdk_color );
      $l->window->set_background( $gdk_color );
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$l->window is undefined, so can't do a set_background on it, and $l->set_background doesn't appear to exist either.