Hi Miglioribirre,
Glad you like the plugin!
The plugin is intended primarily for themes that don’t have built-in responsive navigation, so to use JRSM with a theme like twenty twelve you’ll have to override the default theme styles. No simple way to do that other than manually writing some CSS, as far as I can tell.
Not sure exactly what you mean by targeting the OS, but again I’d look to some custom CSS to get the job done.