You shouldn't normally monkey with the fonts: users pick themes to make things look a particular way. You shouldn't override their choice without a particular good reason. In any case, quoting muppet in DBA5992A-56A5-11D8-8865-000A9580E5E0@asofyet.org:

careful, you tread on ground where the answer is usually "you shouldn't do that, that's what themes are for!"

in general, i've found very little reason to use direct manipulation of fonts as you posted in your example code; i use pango markup for that sort of thing.

you can set markup on a label with the set_markup method, and you can get a button to use this by creating a markup-using button, and putting that into your button.

$button = Gtk2::Button->new;
$label = Gtk2::Label->new;
$label->set_markup ('<big>Hi</big>, <span color="red">this
rocks</span>');
$button->add ($label);
[download]

in my not so humble opinion, that's about a bazillion times easier than dorking about with font descriptions and styles and all that.

Makeshifts last the longest.